Interplanetary Aspects of the Sun-Earth Connection Events on April 1999 and February 2000

E. Echer1, M. V. Alves1, L. A. Balmaceda2, F. L .Guarnieri1, J. C. Santos2, W. D.
Gonzalez1, A. Dal Lago1 and L. E. A. Vieira1
1Instituto Nacional de Pesquisas Espaciais (INPE), Avenida dos Astronautas, 1758,
PO Box 515, 12201-970, São Jose dos Campos, Brazil.
2 Max-Planck-Institut fuer Sonnensystemforschung, Katlenburg-Lindau, Germany
Abstract. There are basically two types of interplanetary ejecta: magnetic clouds (MC) and complex ejecta (CE). In this
work we analyze the geoeffectiveness and the interplanetary aspects of a MC (April 1999, Dst peak = -91 nT) and a CE
(February 2000, Dst peak = -133 nT). Interplanetary propagation conditions, shock strength, ICME axis orientations,
and magnetic field strengths, as well as the total energy transferred by the interplanetary structures to the Earth’s
magnetosphere are described and compared. The magnetic storm profiles are different: smooth and one step for the April
1999 event and complex, with two steps, for the February 2000 event. The energy injection into the magnetosphere is also
of different nature for these events, with higher power and lower integrated values for the February 2000 as compared to
the April 1999 event. These results reflect the Bz profiles, being smooth in the MC and irregular/fluctuating during the
CE.
Keywords: Sun-Earth interaction; geomagnetic indices; magnetic storms; solar wind.
PACS: 94.30.-d; 94.30.Lr; 96.25.Qr.
INTRODUCTION
Solar wind plasma and magnetic field observations are by now routinely used for identification of material in the
interplanetary medium that could be the interplanetary counterparts of coronal mass ejections (CMEs) at the Sun.
Virtually all geomagnetic storms are caused by CMEs or high speed solar wind streams. We can assume that, in
general, there are two classes of interplanetary remnants of coronal mass ejections (ICMEs): magnetic clouds (MC)
and non-magnetic cloud [1, 2]. MCs are identified in the solar wind at ~ 1 AU by the passage of a region with
dimensions around 0.2-0.3 AU crossing the spacecraft or the Earth in ~ 24h. Their main signatures identifiable in
solar wind/interplanetary data are: enhanced magnetic field strengths; smooth rotation in the magnetic field direction
through a large angle; low proton temperature and low plasma beta [3]. The non-MCs are sometimes called complex
ejecta (CE) [4], due to their disordered magnetic field structure. However this name has also been used for the
interaction of two or more ICMEs. Here we will use CE for the magnetic structures resulting of the interaction of
two ICMEs and presenting disordered magnetic field structures.
In this paper we present a comparative analysis between 2 particular events: a typical MC passing Earth during
April 16-17, 1999, from now on referred as MC1999, and a complex ejecta, resulting of the interaction of 2 ICMEs
and lacking a well-ordered magnetic field structure, passing by Earth on February 11-12, 2000, identified from here
as CE2000. The storms caused by these structures are comparable in intensity as measured by the Dst index,
reaching peak values of Dst ~ -100 and -130 nT, respectively. It was also possible to trace their solar origin,
interplanetary propagation conditions and magnetospheric impacts. Their solar origins have been described
previously [5]. Here we present the solar wind plasma and magnetic field data, the characteristics of the ICMEs, and
an analysis of the shocks driven by them. An assessment of the energy transferred to the magnetosphere during the
two events is also presented. This study is a contribution for improving our understanding of the complex Sun-Earth
chain and also of the space weather variability.
DESCRIPTION OF THE EVENTS
Figure 1 presents the solar wind plasma and magnetic field data for the April 16-18, 1999 period. Panels are,
from top to bottom: proton temperature Tp, solar wind speed Vsw, proton density Np, magnetic field azimuth (phi) and
inclination (theta) angles, the magnetic field magnitude B, and GSM (Geocentric Solar Magnetospheric ) magnetic
field components, Bx, By, and Bz, and β. The continuous vertical line delimits the interplanetary shock ‘S’, driven by
the MC. The MC boundaries are delimited by the vertical dotted lines, based on the available signatures of the
magnetic structure. The region between the shock and the first MC boundary is the sheath, where the plasma is
highly turbulent due to shock compression effects. The interplanetary shock was detected at 10:30 UT on April 16,
by ACE. The MC started around ~ 18:00 UT on April 16, lasting until ~19:00 UT on April 17, with duration of ~ 25
h. The solar wind velocity increased, due to the arrival of the shock, from ~ 380 to 470 km/s. Bz deflection was up to
-13 nT and a brief particle density enhancement is also seen, reaching ~ 68 cm-3 within the sheath. The transit speed
of CME (between the solar source and the ACE spacecraft) was estimated in VT ~ 520 km/s, with ICME average
speed of Vsw ~ 400 km/s and maximum pos-shock speed (ICME+sheath) of Vsw ~ 450 km/s [6].
Figure 2 presents the plasma and magnetic field interplanetary data for the ICMEs/complex ejecta observed on
February 10-14, 2000. Panels present the parameters in the same sequence as in Figure 1. The continuous vertical
lines delimit the interplanetary shocks ‘S1’ and ‘S2’. The boundaries of the two ICMEs, ‘ICME1’ and ‘ICME2’, are
indicated by the vertical dotted lines. The ICME1 is abruptly interrupted by the arrival of the second shock ‘S2’.
Two CME-related shock fronts passed the ACE spacecraft on February 11. The first shock, S1, occurred at ~02:14
UT on February 11, accompanied by abrupt changes in Vsw and Np, peaks of ~ 580 km/s and ~14 cm-3, respectively,
and a brief southward IMF with maximum deflection of ~ -7 nT. The second shock front, S2, passed the spacecraft
at ~23:20 UT, February 11, accompanied by a sudden increase in Vsw (up to 640 km/s), in Np (up to 30 cm-3) and in
Bz (~ -18 nT). The beginning of ICME1 was identified at ~16:00 UT, February 11, and abruptly ending at ~20:00
UT, February 11, due to the arrival of shock S2. Its average properties are Vsw~ 420 km/s, |B| ~ 7 nT and transit
speed VT ~ 630 km/s [13]. The ICME2 began around 12:00 on February 12 and lasted until 00:00 UT on February
13. Its average properties were: Vsw ~ 540 km/s; |B|~13 nT; and transit speed ~ 900 km/s [6].
ANALYSIS OF THE EVENTS
Interplanetary shock parameters were determined following the procedure adopted in a previous work [7].
Plasma and magnetic field parameters are averaged in the upstream and downstream regions, and the differences or
ratio between the averaged values are taken as a measure of the shock strength, represented by the density ratio (rN)
and magnetic field ratio (rB). We have also calculated the shock normal (nGSE), the magnetosonic number (MMS)
using the Abraham-Shrauner mixed mode [8], the Alfvén number (MA), and the angle between the shock and the
magnetic field (θBn). US and SI are, respectively, the shock velocity and the sudden impulse observed within the Dst
profile. Shock parameters are summarized in Table 1 for the MC1999 shock and for the two CE2000 shocks.

magnetic field, Bs, configuration.. The MVA was applied to the MC/ICMEs 5-min GSE data in order to determine
their axial orientation [13]. The MC1999 event had its MV axis with an inclination of -16.5o, while the cloud axis
(intermediate variance) had an inclination of 73.5o relative to the ecliptic plane, and an azimuth of 8o in relation to
the Sun-Earth line. This result is in excellent agreement with reference [14] (inclination of 72o and azimuth of 4o).
The ICME1 of CE2000 had an inclination around -5o and an azimuth of -72o, but it had a too short duration to allow
a clear identification of its structure. The ICME2 of CE2000 had an axis inclination of ~18o and azimuth of ~ 152o to
the Sun-Earth line; this indicates that its axis could be close to the ecliptic plane.
CONCLUSIONS
In this work we presented the interplanetary aspects of two different, contrasting Sun-Earth events. The first of
these events was caused by a MC with an intense but smooth and long duration Bs field. The other one was caused
by the interaction of two ICMEs, with intense but irregular and fluctuating Bs fields. The solar wind-magnetosphere
energy coupling, and the resulting geomagnetic storms, as recorded by the Dst index, were of a different nature,
although they had similar intensity. Peak values of energy functions are higher for the complex event, while the total
(integrated) energy deposited in the magnetosphere was higher for the MC event, as shown in Table 2. These
differences between these two events indicate the complexity of the solar wind-magnetosphere coupling and the
need of analyzing the interplanetary structure leading to a magnetic storm to better understand an event. Future
studies should encompass a larger number of events for a better understanding of the basic physics of geomagnetic
storms origin.
ACKNOWLEDGMENTS
This study was partially supported by research fellowship from CNPq (303343/2004-4). Thanks to Brazilian
government agency CAPES for doctoral fellowship and to FAPESP , project number: 02/12723-2, 02/14150-0,
04/14784-4, and 2005/03501-4. Thanks to World Data Center for Geomagnetism-Kyoto for the Dst index, to the
International Solar Terrestrial Physics Project, through WIND, SOHO and ACE teams for high-resolution solar wind
data and to the National Space Science Data Center (NASA/Goddard) for the OMNI data set.
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