Astronomy
&
Astrophysics
A&A 583, A78 (2015)
DOI: 10.1051/0004-6361/201425594
c ESO 2015
Modeling of magnetic cloud expansion
M. Vandas1 , E. Romashets2 , and A. Geranios3
1
2
3
Astronomical Institute of the Czech Academy of Sciences, Boční II 1401, 141 00 Praha 4, Czech Republic
e-mail: vandas@asu.cas.cz
University Park, LSC, Houston, TX 77070, USA
Physics Department, Nuclear and Particle Physics Section, University of Athens, 15771 Ilissia, Greece
Received 29 December 2014 / Accepted 8 August 2015
ABSTRACT
Aims. Magnetic clouds are large interplanetary flux ropes that propagate in the solar wind from the Sun and that expand during their
propagation. We check how magnetic cloud models, represented by cylindrical magnetic flux ropes, which include expansion, correspond to in situ observations.
Methods. Spacecraft measurements of magnetic field and velocity components inside magnetic clouds with clearly expressed expansion are studied in detail and fit by models. The models include expanding cylindrical linear force-free flux ropes with circular or
elliptic cross sections.
Results. From the period of 1995–2009, 26 magnetic clouds were fit by the force-free model of an expanding circular cylindrical
flux rope. Expansion velocity profiles qualitatively correspond to model ones in the majority of cases (81%) and quantitatively in
more than half of them (58%). In four cases an elliptic cross section significantly improved a match between observed and modeled
expansion velocity profiles.
Conclusions. Analysis of velocity components tests magnetic cloud models more strictly and may reveal information on magnetic
cloud shapes.
Key words. solar wind – magnetic fields – magnetohydrodynamics (MHD)
1. Introduction
Magnetic clouds were discovered in late 1970s (Burlaga et al.
1981) and defined as special regions in the solar wind with the
following properties (Burlaga 1991): the magnetic field is higher
and proton temperature lower than in the background, and the
magnetic field vector smoothly rotates through a large angle
(∼180◦). These properties can be easily seen in Fig. 1, which
depicts solar wind observations during the passage of a magnetic cloud, which followed after a big solar eruption, the socalled Bastille Day Event (e.g. Lepping et al. 2001; Mulligan
et al. 2001; Watari et al. 2001). The duration of magnetic cloud
events is on the order of one day, which implies that their sizes
are on the order of 0.1 au. The specific behavior of the magnetic
field and the decrease in temperature led to the supposition that
magnetic clouds were magnetically isolated bodies, i.e., large interplanetary flux ropes propagating in the solar wind.
In the beginning, magnetic clouds were modeled by forcefree configurations (Goldstein 1983; Marubashi 1986; Burlaga
1988), because they have very low plasma β, so magnetic field
dominates. These configurations were quite successful in describing magnetic field rotation within magnetic clouds. Later,
non-force-free models were also developed (e.g., Mulligan &
Russell 2001; Hidalgo et al. 2002; Hu & Sonnerup 2002;
Hidalgo & Nieves-Chinchilla 2012; Hidalgo 2013, 2014) to account for asymmetric magnetic field profiles or plasma quantities. In the present analysis, we deal with linear force-free
models.
Also from the beginning, it has been argued that magnetic
clouds expanded during their propagation in the inner heliosphere (Burlaga et al. 1981; Burlaga 1991). This followed from
their presumably originating at the Sun, but near the Earth they
were much larger than the Sun. Expansion manifests itself as a
smooth decrease in the solar wind velocity during the passage
of a magnetic cloud (Fig. 1). Low proton temperature is also understood as a result of expansion (Burlaga 1991). Ivanov et al.
(1993) attempted to interpret the velocity profiles as a combined
effect of expansion and deceleration by the ambient medium;
they found that both effects are present. The first modeling of
magnetic cloud expansion was done by Osherovich et al. (1993)
and later followed by others (e.g., Marubashi 1997; Hidalgo
2003; Vandas et al. 2006; Marubashi & Lepping 2007). These
models try to fit velocity magnitude profiles in magnetic clouds,
in addition to profiles of magnetic field components. Wu et al.
(2002) did the first investigation of velocity vectors in magnetic
clouds observed near the Earth. In their statistical study, they
found a clear cloud expansion only along the Sun-Earth line.
Vandas et al. (2005b) presented the first comparison of velocity
vectors between observations and their model. It was concluded
that velocity component profiles in magnetic clouds had regular
patterns, but fits with the model were not satisfactory.
Magnetic cloud expansion causes a distortion of the cloud
magnetic field profile, namely a shift of the magnetic field maximum toward the cloud leading boundary (see Fig. 1). Statistical
analysis by Lepping et al. (2003) shows reasonable agreement
between this shift and the assumption of a self-similar expansion. Hidalgo (2003) studied distortion of the magnetic field profile due to expansion in his non-force-free model.
During recent years we have observed an increasing interest in the magnetic cloud expansion. Lepping et al. (2007) conclude that the enhancement of the magnetic cloud front field is
mainly due to compression and a lesser effect to expansion. The
expansion velocity is usually well below the local Alfvén speed
Article published by EDP Sciences
A78, page 1 of 10
A&A 583, A78 (2015)
Bz [nT]
V [km s-1]
0
60
30
0
-30
-60
60
30
0
-30
-60
60
30
0
-30
-60
1000
N [cm-3]
30
1200
800
600
30
20
10
0
107
T [K]
By [nT]
Bx [nT]
B [nT]
60
106
105
104
0 6 12 18 0 6 12 18 0
2000/7/15
16
17
time [UT]
0 6 12 18 0 6 12 18 0
2000/7/15
16
17
time [UT]
Fig. 1. Magnetic cloud of July 15–16, 2000 observed near the Earth.
Measurements (hourly averages) of the solar wind quantities are plotted: magnetic field magnitude B and components B x , By, and Bz in the
geocentric solar ecliptic (GSE) system, velocity V, density N, and proton temperature T . Estimated cloud boundaries are drawn by the vertical
dashed lines. Observations are plotted by the thin line, and the thick line
is a fit by a model (static circular cylindrical flux rope). More details on
it are given in Sect. 2. The cloud is numbered 46.0 in Table 1.
(Lepping et al. 2008). Dasso et al. (2007) used a self-similar expansion to model an observation of a magnetic cloud. Démoulin
et al. (2008) and Démoulin & Dasso (2009) theoretically studied
the causes of magnetic cloud expansion and concluded that the
radial expansion of flux ropes is close to self-similar, resulting
in a nearly linear radial velocity profile, and the expansion rate
is proportional to the radius. Nakwacki et al. (2008) compared
large scale MHD quantities (helicity, magnetic flux, and magnetic energy) in static or expanding flux ropes, and Nakwacki
et al. (2011) investigated changes in these quantities caused by
expansion for a particular magnetic cloud and found agreement
with theoretical predictions. Lynnyk & Vandas (2009, 2010) statistically studied how incorporation of expansion into a model
improved fits of magnetic-field components in magnetic clouds.
Gulisano et al. (2010, 2012) investigated the expansion rates of
magnetic clouds throughout the heliosphere and found that the
self-similar expansion and its distance dependence is consistent
with theoretical expectations. Dalakishvili et al. (2011) assumed
a self-similar expansion of magnetic clouds and derived analytical solutions of the cloud evolution; they show that their solutions are consistent with results from (more general) numerical
simulations.
In the present paper we investigate the behavior of velocity
components in magnetic clouds in more detail. Magnetic clouds
for this study were taken from the magnetic cloud table published on the World Wide Web1 . It is referred to as the www
magnetic cloud table in the present paper.
2. Models and a fitting procedure
Burlaga (1988) suggested using a simple model of a linear forcefree field in a cylindrical flux rope, given by Lundquist (1950).
In cylindrical coordinates it reads as
(1)
Br = 0,
Bϕ = B0 J1 (αr),
(2)
BZ = B0 J0 (αr).
(3)
1
http://wind.nasa.gov/mfi/mag_cloud_S1.html
A78, page 2 of 10
This solution solves the force-free equation curl B = α B,
α = const., Jn are the Bessel functions of the first kind, B0 determines the level of the magnetic field, and α is related to the
flux rope size (radius). The success of this solution at describing
magnetic field rotation satisfactorily in magnetic cloud observations caused this model to be widely used to fit magnetic cloud
events (e.g., Lepping et al. 1990, 2006, 2008) until it could be
regarded as a standard model.
Vandas & Romashets (2003) generalized this model to account for magnetic cloud oblateness. They have found a linear
force-free field in a cylinder with an elliptic cross section. In elliptic cylindrical coordinates, it reads as
∂BZ
1
Bu =
∂v
ε(cosh2 u − cos2 v)
∂BZ
1
Bv = −
∂u
ε(cosh2 u − cos2 v)
BZ = B0
ceh0 (u, −ε/32) ce0 (v, −ε/32)
·
ce20 (0, −ε/32)
(4)
(5)
(6)
Elliptic cylindrical coordinates u, v, and Z are related to
Cartesian ones by
x = c cosh u cos v,
y = c sinh u sin v,
z = Z,
(7)
(8)
(9)
√
where c is a parameter related to oblateness (c = a2 − b2 ,
with a and b being the major and minor semi-axes of a generating ellipse, respectively; contours with u = const. are ellipses
in the xy plane). The parameter ε = (αc)2 is related to the size
and oblateness of the flux rope, ce0 and ceh0 are angular and radial even Mathieu functions of zero order, and B0 determines the
level of the magnetic field.
The component BZ reaches its maximum at the axis (Z) in
both solutions. Usually the boundary of a magnetic cloud is
taken at r0 (Burlaga 1988; Lepping et al. 1990) or u0 (Vandas
& Romashets 2003; Vandas et al. 2005a) where BZ reaches zero,
i.e. where functions J0 or ceh0 have their first roots. It relates
α to the size of the magnetic cloud. For the Lundquist solution,
it is α ≈ 2.4048/r0 where the number stands for the first root
of J0 . For the elliptic flux rope, u0 defines the ellipse with the
semi-axes a and b, cosh u0 = a/c, and α is determined from an
implicit equation and depends not only on the size of the flux
rope, but also on its oblateness (Vandas & Romashets 2003). We
use this location of the boundary in the present analysis.
To model magnetic cloud expansion, Vandas et al. (2006)
used a heuristic approach and replaced the constant parameters α
and B0 in both solutions by time-dependent ones:
α
(10)
α→
1 + t/t0
and
B0 →
B0
·
(1 + t/t0 )2
(11)
These replacements are based on the asymptotic behavior of
an exact solution of MHD equations for a circular cylindrical
flux rope with self-similar expansion, as found by Shimazu &
Vandas (2002; see also Berdichevsky et al. 2003) and inspired
by works of Osherovich et al. (1993, 1995) (who used a less general solution, resulting in a slightly different time dependence in
M. Vandas et al.: Modeling of magnetic cloud expansion
expanding magnetic cloud
their modification of the Lundquist solution). According to the
asymptotic behavior, it is assumed that the flux rope is radially
expanding with velocity
Vr =
r
t + t0
(12)
Vr
Vr
Vr
SUN
V
(b)
spacecraft
time
Vr 0
where r is the radial distance from the flux rope axis (Z) in both
solutions. Such a form ensures that the oblateness of the flux
rope is conserved during expansion. The field is time dependent
but force-free at any instant. A new parameter, the expansion
time t0 , is introduced.
When comparing the standard model (Lepping et al. 1990)
with observations, the model and its relationship to a spacecraft
passage is described by eight parameters: ϑc , φc , r0 , B0 , h, Vsw ,
tm , and p. It is assumed that a cylindrical flux rope with the radius r0 is conveyed by the solar wind with velocity Vsw . The
magnetic field in the flux rope is static and described by the
Lundquist solution (1)–(3) with B0 and α = 2.41h/r0, where
h = ±1 is the magnetic field chirality. The orientation of the
flux rope axis is given by two angles, the inclination ϑc and azimuth φc in the ecliptic plane. Usually the geocentric solar ecliptic (GSE) system (x, y, z) is used for observations near the Earth.
The last parameter p, the impact factor, is related to a minimum
approach of a spacecraft to the cloud axis, and it is described in
Appendix A. If p = 0, it is a central crossing, so the spacecraft
crossed the axis. The time of the minimum approach is given
by tm .
If the flux rope oblateness is included in the model, Eqs. (4)–
(6), two additional parameters appear: oblateness a/b and the
angle ψc , which determines the orientation of the semi-axis b.
The angle ψc is the azimuthal angle between the x axis and
the projection of the semi-axis b to the ecliptic (xy) plane. To
maintain correspondence with the standard model, we define
r0 ≡ b, which also enables the same definition of p to be kept
(see Appendix A). When a/b → 1 (and ψc arbitrary), the model
converges to the standard model. Because this limit is difficult
to calculate numerically, and calculations using Eqs. (4)–(6) are
much more computationally expensive than formulae (1)–(3),
we maintain two models separately with (i) circular and (ii) elliptic cross sections.
The models with expansion (Fig. 2) introduce the last parameter, the expansion time t0 . Modeled velocity magnitude profile
during a passage of an expanding flux rope is shown in Fig. 2b
and yields a nearly linear decrease. Nearly linear decreases in velocity profiles are common in magnetic cloud observations (see
Fig. 1). The expansion time t0 is estimated from the velocity
slope. It determines instant values of α and B0 (10)–(11) in both
solutions (1)–(6), the radial velocity profile by Eq. (12), and instant flux rope radius/semi-axes (with help of the other parameters in the latter case). When t0 → ∞, the models converge to
the static ones, either circular (standard) or elliptic.
Before making a fit, boundaries of a magnetic cloud must
be identified or estimated from observed profiles of solar wind
quantities, which are times t1 and t2 of the spacecraft entry and
exit of the magnetic cloud, respectively (see the dashed vertical lines in Fig. 1). There are many discussions of how to determine magnetic cloud boundaries (see Burlaga 1991; Lepping
et al. 1990, 2006; Gosling 1990; Crooker et al. 1990; Wei et al.
2003), but no general procedure or consensus exists. For fitting purposes, the boundaries usually determine the time interval where solar wind quantities vary sufficiently smoothly and
fit the definition of a magnetic cloud. Subsequently, the interval
may be slightly adjusted to get the best fit possible. It follows
(a)
Vsolar_wind
(c)
time
Fig. 2. a) Spacecraft crossing an expanding circular flux rope. The
spacecraft trajectory is drawn by a straight thin line, the boundary of
the cloud/rope at the spacecraft entry (leading boundary) is the solidline circle, the boundary at the spacecraft exit (trailing boundary) is
the dashed-line circle. The bullet indicates the position of the rope axis.
b) The ambient solar wind and expansion (Vr ) velocities add at the leading boundary and subtract at the trailing one, which results in a decrease
in observed velocity magnitude; the vertical lines indicate the flux rope
boundaries, and their line types match the ones in panel a). c) Radial
velocity Vr is highest near the boundary and drops to zero at the axis. It
yields a characteristic “U” shape profile in the modeled Vr .
from the above description that determining the cloud boundaries includes a subjective element. In the present analysis we
take the times t1 and t2 from the www magnetic cloud table (i.e.,
the start and end times in this table set to whole hours, since we
use hourly data). In a few cases the boundaries were shifted by
a few hours to exclude a peculiar velocity behavior or to obtain
a significantly better fit (most notably in Case 46.0, Fig. 1, as
explained below), that is, to have the times t1 and t2 closer to
the same-magnetic-surface crossings (since the cloud boundary
represents a magnetic surface).
Eleven parameters of the models in total were mentioned
above (ϑc , φc , r0 , B0 , h, Vsw , tm , p, a/b, ψc , and t0 ); some of
them are given in advance and some are free parameters to be
searched by a fitting procedure. We search free parameters by
a non-linear mean square fitting. Powell’s method (Press et al.
2002) is used to minimize a difference between observed and
modeled quantities, that is to say,
⎞2
NB ⎛
⎟⎟
⎜⎜⎜ B(o)
B(m)
i
i
⎜
χ0 =
(13)
⎜⎝ (o) − (m) ⎟⎟⎟⎠ ,
Bi
i=1 Bi
(m)
NB (o)
i=1 Bi − Bi
χB =
,
(14)
NB (o)
i=1 Bi
and
χV =
NV
i=1
(o) (m)
V i − V i
NV (o)
V
i=1
(15)
i
are minimized. The superscripts (o) and (m) denote observed
and modeled values, respectively; NB and NV are numbers of
observed values of the magnetic field components and velocity
magnitude, respectively. This can be NB NV if some observations are missing or not reliable.
We have five free parameters entering the automatic fitting
procedure in the present analysis, ϑc , φc , p, B0 , and Vsw . The fitting procedure needs initial guesses for them. A guess of the axis
orientation is determined from the standard variance analysis of
the magnetic field vectors (Lepping et al. 1990). The parameters B0 and Vsw are taken as the maximum observed field magnitude and the average solar wind velocity within the cloud interval
t1 , t2 , respectively, and p is set to 0.5. The t0 is calculated from
A78, page 3 of 10
A&A 583, A78 (2015)
the velocity magnitude slope in the beginning of the fitting procedure (and does not change). In principle, t0 could be included
as a free parameter in the fitting procedure, but in practice this
approach has not yielded the desired results. Magnetic field component profiles are not very sensitive to oblateness (Vandas et al.
2005a), so we try several values of it (e.g., 1.5, 2, 3, 5) and select one with minimum χB (and not significantly worse χV ). For
simplicity, we set ψc = 0. The chirality h has only two values,
and they can be checked subsequently. Usually one fit is much
better, so the choice is evident. However, in the present analysis,
we took h values from the www magnetic cloud table. The initial (at t = t1 ) radius r0 (or b) and the time tm of the minimum
axis approach are determined from the geometric situation using t1 , t2 , and above given parameters (for the standard model,
tm = (t1 + t2 )/2). Here our approach is different from that of
Lepping et al. (1990, 2006), who regard r0 and tm as free parameters. Then the boundaries of a cloud may not coincide with
t1 and t2 , they may be inside or outside the interval t1 , t2 , and
the measure of coincidence is given by the asymmetry factor in
the www magnetic cloud table. The factor is zero for a perfect
coincidence. We regard t1 and t2 as times of (model) boundary
crossings; i.e., model BZ is zero there, as discussed above. Other
differences concern the solar wind velocity, Vsw , and B0 ; they are
searched for in our case, but the former quantity is prescribed
and the latter one determined after the fitting procedure from
minimum mean square differences of magnetic field magnitudes
in the cited works.
Our fitting procedure runs in three steps: (i) ϑc , φc , and p
(three free parameters) are determined via χ0 minimization; B0
(m) 2
NB
(B(o)
is determined by a least square method from i=1
i − Bi ) =
min; (ii) B0 is added (four free parameters) for determination
and χB minimized; (iii) Vsw is added (five free parameters) for
determination and χB +χV minimized. Steps (i)–(iii) are repeated
several times to check convergence.
Figure 1 shows a fit of the magnetic cloud by the standard
(static) model (i.e., it has a/b = 1 and t0 → ∞ within our model
parameters). The fit yielded ϑc = 13◦ , φc = 41◦ , and p = −0.27.
Passage of the model flux rope by a constant velocity (Vsw ) is indicated by a horizontal line in the velocity plot. While matches
of the magnetic field components are plausible, it is not so for the
magnetic field magnitude. The modeled B profile is symmetric
(intrinsic feature of the model), but the measured profile is highly
asymmetric, because it is affected by a strong expansion of the
magnetic cloud. And this type of disagreement is the reason
expansion is incorporated into models. We later show how the
model with expansion can improve the fit shown in Fig. 1. The
modeled B profile is symmetric and centered within the cloud
interval, because times t1 and t2 are the model cloud boundaries.
The meaning of a (non-zero) asymmetry factor in the www magnetic cloud table is that the modeled B profile is symmetric, but
its maximum is shifted in time within the cloud interval t1 , t2 .
The magnetic field components (such as in Fig. 1) are compared with models in numerous analyses of magnetic cloud observations. More rarely, the velocity magnitude profiles are also
compared. In the present analysis, the models further specify the
radial component of the velocity, Vr , defined in Eq. (12); the
other two components are set to zero. Figure 2c shows how a
model profile of Vr looks like with its “U” shape. We investigate
to what extent modeled and observed radial velocities are similar. The use of “observed” is somewhat inexact because radial
velocities are not directly measured but are instead calculated
from observed velocity components, but the calculations depend
on geometric situation drawn from a model, so the observed radial velocities are not strictly model independent. Nevertheless,
A78, page 4 of 10
25
20
N
15
25
V > 50
q=1-3
q=1-2
q=1
20
N = 121
N = 73
N = 18
N
15
10
10
5
5
0
0
0
0
50 100 150 200 250 300 350
q=1-3
q=1-2
selected
N = 121
N = 73
N = 30
50 100 150 200 250 300 350
V [km/s]
V [km/s]
(a)
(b)
20
15
- V/ t > 50
q=1-3
q=1-2
q=1
N 10
5
0
-700 -600 -500 -400 -300 -200 -100 0
100 200 300 400 500 600 700
- V/ t [km/s per day]
(c)
Fig. 3. Histograms of magnetic clouds from the www magnetic cloud
table. a), b) The magnetic clouds sorted by the velocity difference ∆V,
with a) the ∆V selection criterion (by the arrow) and b) selected cases
displayed. c) The magnetic clouds sorted by the velocity slope ∆V/∆t;
the ∆V/∆t selection criterion is shown by the arrow.
radial velocities usually represent small fractions of the total velocities, so their comparison is a stricter test of the models than
only a comparison of the velocity magnitude would be.
The first comparison of modeled and observed radial velocity profiles has been presented by Vandas et al. (2009), who
used the circular model with expansion. Here we make a more
comprehensive analysis and examine whether the introduction
of oblateness into the model can improve radial-velocity profile
matches.
3. Data analysis
We have analyzed magnetic cloud observations from the www
magnetic cloud table; it is an extended version of Table 1 in
Lepping et al. (2006). The table lists 121 magnetic clouds for
the period 1995–2009 with parameters derived from fits by the
standard model. The quality of their fit is marked by 1–3 (1 is
the best). We only considered clouds of quality levels 1 and 2:
there are 73 cases. Figure 3a is a histogram of clouds from the table grouped according to the quality and velocity difference ∆V
within the cloud interval, that is, the difference between the maximum and minimum solar wind velocities, ∆V = Vmax − Vmin .
We used hourly averages of solar wind data from
OMNIWeb2 in our work. The analysis was subsequently restricted to clouds with well-expressed expansion (see Fig. 2b):
sufficiently large velocity decrease and slope. We considered
clouds with ∆V > 50 km s−1 (Fig. 3a shows this selection).
The histogram in Fig. 3c displays clouds grouped according
to velocity slopes ∆V/∆t. The slopes were determined from a
linear fit of the velocity magnitude profiles within the cloud
2
http://omniweb.gsfc.nasa.gov
0
-20
12 18 0 6 12 18 0 6 12
2001/4/21 22
23
time [UT]
104
50
25
0
-25
-50
12 18 0 6 12 18 0 6 12
2001/4/21 22
23
time [UT]
Fig. 4. Magnetic cloud of April 22, 2001. It has number 58.0 in Table 1.
The format is similar to Fig. 1, and only the radial velocity profiles Vr
are added: the observed radial velocity is plotted by the thin line, and
the thick line is the profile following from the model (expanding circular
cylindrical flux rope).
interval. Positive slopes (contraction) are not rare and indicate the interaction of magnetic clouds with their surroundings (other ejecta or fast streams). These cases often have nonmonotonous velocity magnitude profiles. Occasionally the slope
reaches nearly 700 km s−1 per day in both signs (Fig. 3c; these
are the Bastille Day Event, Fig. 1, and the cloud on 22 July 2004
in the considered table). Our analysis included only cases with
−∆V/∆t > 50 km s−1 per day (Fig. 3c shows this selection). Low
|∆V/∆t| values have slowly expanding clouds, but they may also
result from non-monotonic velocity magnitude profiles. After
applying all the above given criteria, 30 magnetic clouds remained (Fig. 3b), which are listed in Table 1.
4. Results
Using our fitting procedure with the circular model with expansion, the cloud parameters were determined, and the radial velocity profiles (observed and modeled) were visually inspected.
The results of our visual Vr comparison are summarized in the
last column of Table 1. Of course, these judgments are somewhat
subjective, but below we explain their meanings and demonstrate
them in figures.
We mean by “reasonable fit” those cases where the observed
Vr profile exhibits the “U” profile, and it is at least partly met
quantitatively by the model profile. Figure 4 shows one example
where the magnetic field components, velocity magnitude, and
radial velocity are met quite well.
Figure 5 shows the magnetic cloud from the Bastille Day
Event, which was already displayed in Fig. 1 but now fit by
the cylindrical model with expansion. The introduction of expansion does not automatically mean that fits of magnetic field
components improve. Some may improve, but others are better
matched by the static model. What improves is the match of the
magnetic magnitude profile, and the velocity magnitude profile
is fit, sometimes very well, but this is not the current case. As
the figure demonstrates, expansion causes an asymmetry in the
magnetic field’s magnitude profile, and the maximum is shifted
toward the leading boundary, that is, the field increase is shorter
and its decline is longer.
0 6 12 18 0 6 12 18 0
2000/7/15
16
17
time [UT]
V [km s-1]
800
600
30
N [cm-3]
0
60
30
0
-30
-60
60
30
0
-30
-60
60
30
0
-30
-60
1000
20
10
0
107
106
T [K]
B [nT]
30
1200
105
104
200
100
0
-100
-200
0 6 12 18 0 6 12 18 0
2000/7/15
16
17
time [UT]
Vr [km s-1]
Bz [nT]
-20
20
10
T [K]
0
20
0
105
Vr [km s-1]
By [nT]
-20
20
300
30
Bx [nT]
0
350
60
By [nT]
Bx [nT]
0
20
400
Bz [nT]
10
N [cm-3]
B [nT]
20
V [km s-1]
M. Vandas et al.: Modeling of magnetic cloud expansion
Fig. 5. Magnetic cloud of July 15–16, 2000, already displayed in Fig. 1
(number 46.0 in Table 1), but fit by the expanding circular cylindrical
flux rope. The format is the same as in Fig. 4.
We could not get a fit of this case with reasonable parameters when using t1 and t2 from the www magnetic cloud table.
The event is also treated by Lepping et al. (2001), who present
a different time interval, which we used here in Figs. 1 and 5.
One can notice in Table 1 a large difference between the axis
orientations following from our fitting and the values from the
www magnetic cloud table (numbers in parentheses). It reports
a flux rope with a relatively large inclination. A similar axis orientation is given in Lepping et al. (2001) with a relatively large
asymmetry (33%).
Our fitting procedure converges to a low inclined flux rope
in both static and expanding model cases (with the initial radius
around 0.1 au), and the resulting fits are seen in Figs. 1 and 5.
Mulligan et al. (2001) also deduce a low-inclined flux rope. The
magnetic-field and velocity magnitude profiles are not fit very
well. The case is an extreme, with an extraordinary velocity decrease. The fit of the magnetic field magnitude suggests that the
model cloud should expand more slowly, but the fit of the velocity magnitude indicates a faster expansion. These requirements
are in contradiction, and the fit represents a compromise. An introduction of oblateness does not help in this case and makes the
fit of the velocity magnitude even worse. Apparently our simple
model cannot adequately describe in detail the evolution of this
cloud. Both observed and modeled Vr have “U” profiles, but they
do not agree quantitatively, so we label this case as ““U” shape
expressed, fit only qualitative”.
Figure 6a shows the first entry in Table 1, the magnetic cloud
of February 8, 1995, fit by an expanding model with a circular
cross section. Inspecting the Vr panel, we see that the radial velocity profiles are only partly met. However, an introduction of
oblateness significantly improves this fit (see Fig. 6b). The reason can be deduced from parameters given in Table 1, and it
is also demonstrated in Fig. 7. Axis orientations following from
these two fits are relatively close together, so Fig. 7 plausibly displays a spatial relationship of the models. However, the spacecraft crosses boundaries of the circular and elliptic magnetic
clouds in different places owing to different cloud-axis orientations. If the axis orientations (given by ϑc and φc ) following from
the two fits were the same, then the spacecraft trajectory would
pass through the mutual crossing points of the circular and elliptic cloud boundaries. It nearly occurs in Case 11.0, discussed
A78, page 5 of 10
A&A 583, A78 (2015)
Table 1. List of 30 analyzed magnetic clouds.
Va
[km s−1 ]
∆V
[km s−1 ]
−∆V/∆t
[km s−1 /d]
ϑc
[deg]
φc
[deg]
p
a/b
q
Result for Vr comparison
19.0
407
53
62
reasonable fit
elliptic shape reasonably improves the fit
305
84
70
–
2
“U” shape expressed, fit only qualitative
11.0
96/12/24
32.5
349
90
57
–
1.8
1
Vr < 0
negative Vr fixed by an elliptic shape
12.0
97/01/10
21.0
436
54
64
–
5.0
1
reasonable fit
reasonable fit
22.0
97/09/22
16.5
419
107
155
–
2.0
2
Vr < 0
negative Vr fixed by an elliptic shape
28.0
98/01/07
29.0
383
63
51
–
1
reasonable fit
34.0
98/06/24
29.0
457
123
72
–
2
“U” shape expressed, fit only qualitative
36.0
98/09/25
27.0
643
157
134
–
2.0
2
Vr < 0
negative Vr fixed by an elliptic shape
46.0
00/07/15
12.8
933
285
688
–
2
“U” shape expressed, fit only qualitative
49.0
00/08/12
23.0
576
83
61
–
2
reasonable fit
54.0
00/11/06
19.0
527
140
161
–
2
“U” shape not expressed, unusual B
55.1
01/03/19
19.0
407
64
85
–
1
reasonable fit
56.0
01/04/04
11.5
680
80
120
0.57
0.16
(0.49)
0.46
(0.71)
0.21
−0.06
(0.47)
−0.06
−0.58
(0.11)
−0.77
0.01
(−0.03)
−0.02
(0.02)
0.33
(0.23)
−0.19
0.16
(−0.57)
−0.29
(0.17)
0.03
(0.01)
0.61
(0.19)
0.60
(0.19)
0.88
(0.86)
2
27.0
124
85
(100)
108
(96)
109
126
(82)
232
227
(247)
38
143
(151)
35
(26)
167
(151)
179
176
(212)
42
(44)
103
(95)
146
(114)
193
(213)
279
(281)
–
2.0
95/04/03
−14
−14
(18)
0
(−22)
35
30
(26)
−16
−15
(1)
49
56
(60)
49
(55)
25
(21)
37
23
(46)
4
(45)
15
(3)
0
(−9)
−16
(−34)
0
(8)
–
1
reasonable fit
57.0
58.0
01/04/12
01/04/22
10.0
24.5
632
357
73
54
81
55
cannot evaluate, non-monotonous V
reasonable fit
11.0
628
79
203
–
2
Vr < 0, not fixed by an elliptic shape
60.0
01/05/28
22.5
452
84
91
–
1
reasonable fit
66.0
02/04/18
22.0
472
69
83
–
1
“U” shape expressed, fit only qualitative
68.0
02/05/19
19.5
432
83
77
−0.10
(−0.05)
−0.43
(−0.39)
−0.17
(−0.37)
0.80
(0.53)
0.85
(0.95)
2
2
01/04/29
270
(293)
111
(119)
72
(49)
348
(318)
230
(256)
–
59.0
−56
(−78)
4
(31)
0
(−12)
−5
(−27)
−1
(2)
–
1
Vr < 0, not fixed by an elliptic shape
73.0
76.0
77.0
03/03/20
03/08/18
03/11/20
10.5
16.8
15.5
659
438
582
110
54
177
101
64
240
–
1
2
2
cannot evaluate, non-monotonous V
cannot evaluate, non-monotonous V
reasonable fit
78.0
04/04/04
36.0
425
138
82
–
2
reasonable fit
82.0
04/11/08
13.3
678
99
138
–
2
“U” shape expressed, fit only qualitative
84.0
04/11/10
7.5
717
81
253
–
2
“U” shape expressed, fit only qualitative
85.0
05/05/15
16.5
860
216
220
–
2
Vr < 0, not fixed by an elliptic shape
86.0
05/05/20
22.0
452
60
70
–
2
reasonable fit
87.0
89.0
05/06/12
05/07/17
15.5
12.5
475
424
54
57
78
109
–
2
2
cannot evaluate, non-monotonous V
reasonable fit
95.0
06/08/30
17.8
410
65
98
–
2
“U” shape not expressed, unusual B
Code
Date
∆T
[h]
1.0
95/02/08
2.2
−76
(−76)
58
(69)
0
(−5)
−45
(−59)
49
(67)
68
(59)
140
(217)
8
(76)
34
(47)
9
(37)
132
(94)
305
(221)
−0.06
(−0.03)
0.12
(0.48)
−0.41
(−0.27)
−0.66
(−0.41)
−0.48
(−0.75)
0.01
(0.34)
−14
(−41)
5
(−8)
79
(79)
208
(223)
−0.24
(−0.44)
0.70
(0.51)
Notes. Code (number) identifies the event in the www magnetic cloud table from which the quality flag q, the duration ∆T of the event in hours,
and values in parentheses were taken. Va is the averaged solar wind velocity, ∆V and −∆V/∆t are defined in Sect. 3, d is day (time unit), ϑc and
φc are the inclination and azimuthal angle of the cloud axis in the GSE coordinates, respectively, p is the impact parameter (ratio of the closest
spacecraft approach to the cloud axis and the cloud radius or semi-minor axis; the approach can be from two sides, hence p has a sign; more on p
and its sign is in Appendix A), a/b is the oblateness (the symbol “–” means that the model with the circular cross section was used).
A78, page 6 of 10
M. Vandas et al.: Modeling of magnetic cloud expansion
0 6 12 18 0 6 12 18 0
1995/2/8
9
10
time [UT]
104
50
25
0
-25
-50
0 6 12 18 0 6 12 18 0
1995/2/8
9
10
time [UT]
(a)
Bz [nT]
T [K]
By [nT]
0
105
V [km s-1]
400
300
20
N [cm-3]
10
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
500
10
0
105
T [K]
300
20
10
Vr [km s-1]
B [nT]
400
N [cm-3]
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
20
500
Bx [nT]
V [km s-1]
10
Vr [km s-1]
Bz [nT]
By [nT]
Bx [nT]
B [nT]
20
0 6 12 18 0 6 12 18 0
1995/2/8
9
10
time [UT]
104
50
25
0
-25
-50
0 6 12 18 0 6 12 18 0
1995/2/8
9
10
time [UT]
(b)
Fig. 6. Magnetic cloud of February 8, 1995 (number 1.0 in Table 1) fit by expanding cylindrical flux ropes with a) circular or b) elliptic cross
sections. The format of panels is the same as in Fig. 4.
spacecraft trajectory
Xc direction
B
C
E
Yc
Fig. 7. Geometric situation depicting a spacecraft passage through modeled magnetic clouds in Case 1.0. Two magnetic cloud systems are superposed, one (with the origin C) related to an expanding circular magnetic cloud and Fig. 6a, and the second one (with the origin E) related
to an expanding elliptic magnetic cloud and Fig. 6b. The systems are
aligned with the Yc axes and the Xc Yc projections of the spacecraft trajectory (shown by the labeled straight vertical line). The Xc axes (not
shown) are parallel to this vertical line and cross either the point C,
or E. The solid-line circle and ellipse are magnetic-cloud cross sections
at the spacecraft entry, while the dashed-line shapes display them at the
spacecraft exit. The sense of the magnetic field rotation is shown by a
bent arrow, and there is also BZ component pointing toward the viewer
(i.e., the field is lefthanded, h = −1).
later and shown in Fig. 9, where the cloud axis directions resulting from the two fits only differ by 15◦ , unlike the current case
with the value of 38◦ . (Another reason for the boundary crossing discrepancy can be different times t1 and/or t2 of the two fits,
but it is not the case for the clouds displayed in Figs. 7 and 9;
compare, for example, these times in Figs. 6a and b.) Returning
to the current case, 1.0, impact parameters p are quite different
for the two fits, the elliptic fit brings the spacecraft much closer
to the cloud axis, so Vr becomes lower and is more consistent
with the data (Fig. 6b versus Fig. 6a).
Figure 8a displays the magnetic cloud of September 22, 1997
(number 22.0 in Table 1) fit by an expanding circular cylindrical flux rope. While the fits of magnetic field components
and the field and velocity magnitudes are reasonable (but note
a large difference in parameters in Table 1 between this fit and
values in parentheses taken from the www magnetic cloud table), it is not true for the radial velocity. Their difference is very
large. Moreover, the observed radial velocity is negative (therefore the label “Vr < 0” in Table 1) in the center, which is strange
because it should indicate a compression. The model’s radial velocity is always positive (or zero at the axis). The negative observed Vr indicates that something is wrong with the correspondence between the cloud and the model. We again tested that the
oblateness of the cloud could improve the situation. Figure 8b
shows how the fit improves when oblateness is introduced into
our model. Here the fitting procedure brings the axis orientation
closer to the values in parentheses, and the large change in the
axis orientation and impact parameter yields a different geometric situation and makes Vr more consistent with the data. We
consider this fit to be reasonable, and it is the meaning of words
“negative Vr fixed by an elliptic shape” in Table 1.
Another case with negative Vr fixed is the magnetic cloud of
December 24, 1996 (Case 11.0 in Table 1). It has been treated in
Vandas et al. (2009), who fit it by an expanding circular cylindrical flux rope. Figure 4 there shows this fit with the negative
Vr profile. The modeled axes of circular and elliptic clouds have
similar directions (see Table 1), so the clouds are suitable for superposed analysis (Fig. 9). These two fits have opposite signs of
p, so the model axes are located at opposite sites with respect to
the spacecraft trajectory (see Fig. 9). Assuming that the correct
axis follows from the elliptic fit (origin/axis in point E) and the
cloud expands from the axis, then Vr (shown in Fig. 9) is directed
away from E (Vr > 0), but toward C (Vr < 0).
Fit of the cloud from April 29, 2001 (Case 59.0 in Table 1)
also yields a negative Vr (Fig. 10). There is a very good match
of the magnetic field components and velocity magnitude, but
still a large difference in the Vr profiles, and observed Vr is negative. The model with oblateness does not help. The case is labeled as “Vr < 0, not fixed by an elliptic shape”. There are
three such cases in Table 1. Another possibility for explaining
A78, page 7 of 10
A&A 583, A78 (2015)
0
-100
12 18 0 6 12 18 0 6 12
1997/9/21 22
23
time [UT]
(a)
V [km s-1]
By [nT]
Bz [nT]
104
100
N [cm-3]
Bx [nT]
105
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
12 18 0 6 12 18 0 6 12
1997/9/21 22
23
time [UT]
400
200
40
30
20
10
0
106
T [K]
200
40
30
20
10
0
106
10
600
105
Vr [km s-1]
B [nT]
V [km s-1]
400
N [cm-3]
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
12 18 0 6 12 18 0 6 12
1997/9/21 22
23
time [UT]
20
T [K]
10
600
Vr [km s-1]
Bz [nT]
By [nT]
Bx [nT]
B [nT]
20
104
100
0
-100
12 18 0 6 12 18 0 6 12
1997/9/21 22
23
time [UT]
(b)
Fig. 8. Magnetic cloud of September 22, 1997 (number 22.0 in Table 1) fit by expanding cylindrical flux ropes with a) circular or b) elliptic cross
sections. The format of panels is the same as in Fig. 4.
C E
Yc
Bz [nT]
B
Fig. 9. Geometric situation depicting a spacecraft passage through modeled magnetic clouds related to Case 11.0 in Table 1 (magnetic cloud of
December 24, 1996). The format is the same as in Fig. 7. In addition,
the direction of the radial velocity Vr for the elliptic model is shown.
The cloud is righthanded (h = +1).
(removing) the observed negative radial velocity is that the cloud
axis is bent. We have suggested it in Vandas et al. (2009) but did
not treat it in detail. Such an analysis goes beyond the scope of
the present paper where models with straight axes are used. We
shall treat them in a paper under preparation on toroidal magnetic clouds.
Figure 11a shows the magnetic cloud of January 10, 1997
(number 12.0 in Table 1) fit by an expanding circular cylindrical
flux rope. The fit of the magnetic field and velocity is reasonable,
as is the radial velocity. The magnetic-field magnitude profile is
very flat, and we have suggested earlier (Vandas et al. 2006) that
it may indicate an oblate shape of the magnetic cloud. In this
paper the cloud was fit by an elliptic flux rope. Within the scope
of the present paper we examined how this fit changes radial
velocity profiles. The result is shown in Fig. 11b. We see that the
quality of the Vr fit does not change (does not get worse), but the
fit of the field magnitude did improve.
A78, page 8 of 10
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
0 6 12 18 0 6 12 18 0
2001/4/28
29
30
time [UT]
V [km s-1]
10
600
400
10
N [cm-3]
Vr
By [nT]
Bx [nT]
Xc direction
800
0
106
T [K]
B [nT]
20
105
104
200
100
0
-100
-200
0 6 12 18 0 6 12 18 0
2001/4/28
29
30
time [UT]
Vr [km s-1]
spacecraft trajectory
Fig. 10. Magnetic cloud of April 29, 2001 (number 59.0 in Table 1) fit
by an expanding circular cylindrical flux rope. The format is the same
as in Fig. 4.
Some cases could not be analyzed by the present model as
can be inferred from Table 1. Our selection criteria did not exclude all non-monotonous velocity profiles. A few cases have
unusual B profiles, with B increasing toward the trailing edge
(i.e., the opposite of what is expected by the model). When these
features dominate, it makes no sense to use the present model,
because its assumptions are violated.
5. Summary and conclusions
We have analyzed 30 magnetic clouds with clearly expressed
expansion in detail: 26 of them were fit by the linear force-free
model of an expanding circular or elliptic cylindrical flux rope,
and the remaining four violated assumptions in the model by
having strongly non-monotonous velocity profiles. Radial velocities in 21 cases (81% of 26) behave as expected and were quantitatively compatible with the model in 15 cases (58%). The observed negative radial velocities were present in six cases, three
M. Vandas et al.: Modeling of magnetic cloud expansion
T [K]
105
10
4
By [nT]
0
106
3
0 6 12 18 0 6 12 18 0
1997/1/10
11
12
time [UT]
0
-100
0 6 12 18 0 6 12 18 0
1997/1/10
11
12
time [UT]
Bz [nT]
10
100
V [km s-1]
400
300
80
40
0
106
0 6 12 18 0 6 12 18 0
1997/1/10
11
12
time [UT]
(a)
500
N [cm-3]
10
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
600
105
T [K]
40
Bx [nT]
300
80
20
104
Vr [km s-1]
400
30
B [nT]
500
N [cm-3]
10
0
20
10
0
-10
-20
20
10
0
-10
-20
20
10
0
-10
-20
600
V [km s-1]
20
Vr [km s-1]
Bz [nT]
By [nT]
Bx [nT]
B [nT]
30
103
100
0
-100
0 6 12 18 0 6 12 18 0
1997/1/10
11
12
time [UT]
(b)
Fig. 11. Magnetic cloud of January 10, 1997 (number 12.0 in Table 1) fit by expanding cylindrical flux ropes with a) circular or b) elliptic cross
sections. The format of panels is the same as in Fig. 4.
of which were converted into expected positive values by the
model including the flux rope oblateness. This model changed
the parameters of the flux ropes, meaning they changed the geometric situation of the spacecraft passage through the clouds and
usually brought the spacecraft closer to the cloud axis, which
removes the observed negative radial velocity profiles. The remaining three cases have relatively large impact factors. This
might be the reason for the discrepancy, but we cannot exclude
that the discrepancy might be explained by a flux rope curvature
(future task). There were two cases where the radial velocities
even behave differently qualitatively, but they were accompanied
by peculiar magnetic field magnitude profiles.
To conclude, radial velocity profiles qualitatively correspond
to model ones in the majority of cases and quantitatively in more
than half of them. In general, analyses of velocity components
test models more strictly and may reveal information about magnetic cloud shapes.
Acknowledgements. We acknowledge the use of data from OMNIWeb and PIs
who provided them. This work was supported by projects 205/09/0170 and
14-19376S from GA ČR and by the AV ČR grant RVO:67985815.
Appendix A: Impact parameter and its sign
Let us assume that a spacecraft is located in the origin of a
GSE system (x, y, z). A magnetic cloud approximated by a
cylindrical flux rope is traveling radially from the Sun. Its axis
crosses the ecliptic plane in a point (ignoring a singular case
when ϑc = 0◦ exactly) that is conveyed by the solar wind parallel to the x axis; in other words, the point has its y coordinate, YG , constant. Following Lepping et al. (1990), a magnetic cloud system is introduced, Zc , the axis of which coincides
with the cloud axis. If denoting a unit vector in the direction
of the spacecraft trajectory within the magnetic cloud system
by S, then the Yc axis is defined by a unit vector in its direction, Ŷ c = ( Ẑ c × S)0 , where Ẑ c is similarly a unit vector in the
Zc axis direction. The Xc axis completes the system by its unit
vector X̂ c = Ŷ c × Ẑ c . These unit vectors can be easily calculated in the GSE system when we realize that S = (1, 0, 0) and
Ẑ c = (cos ϑc cos φc , cos ϑc sin φc , sin ϑc ). The spacecraft in the
magnetic cloud system moves along a trajectory that also has a
constant Yc coordinate, denoted by Y0 in Lepping et al. (1990).
Here, |Y0 | is the closest distance of the spacecraft from the cloud
axis, and the impact parameter p = Y0 /r0 where r0 is the cloud
radius, so the sign of p is the sign of Y0 , and its absolute value
is the relatively closest distance. When the cloud is elliptic, we
define p = Y0 /b, where b is the minor radius. If the cloud is
expanding, the values of r0 or b are taken at the spacecraft entry
into the cloud.
The angle δ between the x and Xc axes is given by
δ = arccos cos2 ϑc sin2 φc + sin2 ϑc . These two axes therefore
have similar directions for low inclined flux ropes (ϑc ≈ 0◦ ), the
axes of which are roughly perpendicular to the Sun-Earth line
(φc ≈ 90◦ or 270◦ ). In a very crude approximation, these conditions hold for the clouds shown in Figs. 7 and 9. Values of Y0
and YG are related by Y0 = −YG sin ϑc / cos δ.
Lepping et al. (1990) do not specify explicitly the orientation of the vector S in the text, but their Fig. 1 suggests that it is
in the direction of the spacecraft motion through the cloud. Our
vector S has this direction. However, comparison of their magnetic cloud parameters with data lead us to the conclusion that
their S goes in the opposite direction, against the spacecraft motion, so the values of p in parentheses in Table 1 are defined as
p = −CA/100, where CA is the parameter from the www magnetic cloud table related to the impact parameter. This is supported by, e.g., Figs. 7 and 6. The cloud has a positive impact
parameter (see Table 1), so Y0 is also positive, in accord with
the geometric situation given in Fig. 7. The cloud is lefthanded,
as it is indicated in this figure, so the spacecraft should observe
positive B x , which is confirmed by Fig. 6. When p is opposite,
as suggested by the www magnetic cloud table, the observed B x
should be negative, which is not the case.
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