A&A 400, 729-736 (2003)
E. Kirsch - U. Mall
Max-Planck-Institut für Aeronomie, 37191 Katlenburg-Lindau, Germany
Received 21 February 2002 / Accepted 10 December 2002
The present study deals with suprathermal proton ( E/q=6.5-225 keV/e) and -particle bursts measured by the WIND-SMS experiment in the interplanetary space. They reach up to 5-20 times the solar wind speed and last from a few minutes up to 30 min. Measurements obtained simultaneously by the Solar Wind Ion Composition Sensor SWICS ( E/q=0.5-31.5 keV/e) were also available for this study, as well as magnetic field and particle data recorded by ACE near the Libration point L1 and the IMP8-S/C near the Earth. In order to exclude particles escaping from the magnetosphere or accelerated by the Earth's bow shock, interplanetary shocks, coronal mass ejections and corotating interaction regions, we selected ion bursts which were associated with a distinct decrease in the interplanetary magnetic field magnitude and with changes in the azimuthal and tangential field direction. Such changes have been known for a long time as magnetic holes or field depressions. We interpret these signatures as a manifestation of a reconnection process in the interplanetary space near the heliospheric current sheet at about 1 AU distance from the Sun and show for the first time that thermal particles can be accelerated up to 100 keV/e. The suprathermal particles are most likely accelerated in the electric field of the X-line. Inductive electric fields caused by changes in the field magnitude could also be responsible for the particle acceleration.
Key words: solar wind
|Figure 1: From top to bottom are shown: SWICS (0.5-31.5 keV/e), STICS (6.5-225 keV/e) ion count rates, M/Q = mass/charge ratio, count rate in the sectors 0-15, wind velocity, wind thermal velocity, , B = magnetic field magnitude together with the ratio , , field direction. Shadowed are suprathermal (6.5-225 keV/e) particle bursts. Two distinct decreases in the magnetic field magnitude B and suprathermal bursts appeared at 3:10 and 13:10 UT. The STICS count rates consist of H+, He++ and heavier ions. The intensity increases of (0.5-31.5 keV/e) ions and the small increase in by 30-50 km s-1 at 1:00 and 12:00 UT seem to also be caused by the reconnection process. and indicate that the heliospheric current sheet was crossed. The WIND-S/C was near the Earth.|
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|Figure 2: Simultaneously obtained solar wind ion velocity, temperature, density, energetic ions (47-85 keV) and magnetic field measurements of the ACE-S/C, located in the libration point L1 and IMP8, magnetic field and ion measurements (>15 keV). The solar wind and the (47-85 keV) ions are obviously accelerated by a similar magnetic field structure, however, about 1 h earlier than near the Earth.|
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|Figure 3: The same as Fig. 1 for 16-17 March 1995. A two-step decrease of B and particle bursts were measured between 16 March, 20:00 UT and 17 March, 4:00 UT 1995. The E/q=6.5-225 keV/e ions showed an intensity increase together with the second B-decrease. Also presented are the thermal velocity and the density of the solar wind plasma. The WIND-S/C was in the libration point L1.|
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In Fig. 2 measurements of the ACE-S/C, located in the libration point L1, are depicted together with magnetic field magnitude measurements of IMP8 ( , ) and ion measurements of IMP8 for the same event. The panels display (from top to bottom) the solar wind velocity measured by ACE (located in the libration point), temperature in Kelvin, density, flux of 47-85 keV ions, magnetic field magnitude B and the components Bx, By, Bz. In the lower two panels the magnetic field magnitude B and >15 keV-ions measured by IMP8 near the Earth are displayed. The magnetic field decreases and the production of ions (45-85 keV), as well as the density and temperature variation of the solar wind plasma, appear about 1 hour earlier in the libration point than near the Earth. Thus, one could conclude that the reconnection process in the magnetic field lasted at least 1 hour or that it started again when the solar wind with the magnetic field reached the Earth.
Figure 3 presents two bursts observed by WIND in the libration point L1 from 16-17 March 1995. One sees two decreases in the field magnitude B and changes in the tangential and azimuthal field direction between 16 March, 20:00 UT and 17 March, 5:00 UT 1995. The SWICS sensor shows a broad and a small burst. Only the second one also consisted of protons which reached about 10 times the solar wind speed, as can be seen from the STICS sensor. The sector measurements let us conclude that the first burst observed by STICS probably started antisunward from the WIND-S/C, since the intensity increase was observed in sectors 0-6 and 12-15 which indicate a particle population trapped on field lines. The gyroradii of 10, 50 and 100 keV protons in a 5 nT field are 2900, 6400 and 9100 km, respectively. The second burst (Fig. 3) appeared sunward from WIND, since the flux increase was measured in the sectors 6-15 which point mainly in a sunward direction. From Fig. 3 it can be seen that the plasma density increases during the reconnection process but not the thermal velocity of the ions. A comparison of the magnetic field measurements by WIND located in the libration point L1 and IMP8 near the Earth revealed again a delay of 1 hour (not shown as a figure).
The third example (Fig. 4) shows again two decreases in the magnetic field magnitude on 23 April 1997 between 10:12 UT and 13:12 UT. The WIND-S/C was in the libration point L1. The SWICS sensor measured a broad single peak during that interval, while STICS detected a single short burst at the end of that interval. The plasma density, but not the thermal velocity, increased during this reconnection event. The sector measurements indicate isotropy for the burst at 13:00 UT and an anistropy for the small burst at 0:30 UT which is also caused by a small B-decrease. The fluxes in sectors 9 and 10 result from solar particles and 4He+ as the M/Q panel of Fig. 4 demonstrates. IMP8 measurements from 23 April 1997 are not available for a comparison.
In Fig. 5 a magnetic field depression is shown which was classified by
Zurbuchen et al. (2001) as a "microscale magnetic hole'' observed by ACE
near the heliospheric current sheet. The authors suggest that such
holes are formed by reconnection close to the Sun. Figure 5 presents,
from top to the bottom, thermal (0.5-31.5 keV/e) and suprathermal
(6.5-225 keV/e) ions
measured by WIND near the heliospheric current sheet
in the libration point L1. Then the M/Q ratios, the sector
measurements, the solar wind velocity, thermal velocity and density
The last three panels show magnetic field magnitude B, together with
the tangential and azimuthal field direction.
It can be seen that this magnetic field depression is associated
with an increase in the thermal ion (0.5-31.5 keV/e) count rate, the solar
the thermal velocity and density. Suprathermal
particles (6.5-31.5 keV/e) appear
as a short-lived peak of isotropic distribution
and a velocity of 12 times the solar wind velocity at the end of the
long lasting field depression. Simultaneously,
the magnetic field magnitude shows a small depression of 5 nT and
the azimuthal direction changes .
All particle bursts presented in Figs. 1-5 were also observed by the WIND 3D Plasma experiment
(R. P. Lin et al., see CDA Web Wi_K03DP).
|Figure 4: The same as Fig. 1 for 23 April 1997. Again, two decreases of B appeared at 10:33 UT and 13:12 UT. The low energy plasma (E/q=0.5-31.5 keV/e) started to increase in its intensity during the first B-decrease. Only the second decrease showed suprathermal ions (E/q=6.5-225 keV/e). Also presented are the thermal velocity and the density of the solar wind plasma. The WIND-S/C was in the libration point L1.|
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|Figure 5: The same as Fig. 1 for 9 April 1998. The magnetic field depression is associated with increases in the SWICS (E/q = 0.5-31.5 keV/e) ion count rate, a short-lived burst of suprathermal (E/q = 6.5-225 keV/e) ions, as well as increases of the solar wind velocity, its thermal velocity and density. The WIND-S/C was in the libration point L1.|
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In Fig. 6 the four bursts are shown once more for proton fluxes (condition
mass/charge < 1.5). The proton count rates could then be converted into
calibrated energy spectra by using the equation
The calibrated energy spectra of the 4 bursts are shown in Fig. 7.
|Figure 6: The four bursts are shown for protons only ( ).|
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The question of whether reconnection processes can occur in the interplanetary
magnetic field has been discussed by various authors (Schindler 1972;
Bavassano et al. 1976). Turner et al. (1977) considered magnetic holes as a
new kinetic scale phenomenon. Some of them result from magnetic merging.
McComas et al. (1994) identified the reconnection
process ahead of a coronal mass ejection. Winterhalter et al. (2000)
described the observed solar wind large-scale magnetic holes as a result
of the mirror mode instability, while the plasma was characterized by
|Figure 7: Calibrated energy spectra of the four proton bursts, averaged over 0.1, 0.15 and 0.2 days, respectively.|
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The selection criteria for the bursts here presented
(strong time correlation between the occurrence of bursts and
the decreases in the magnetic
field magnitude) allowed us to exclude bursts escaping from the geomagnetic
field, solar protons and bursts which could be accelerated
in the interplanetary space by CIRs or shocks.
We presented strong evidence that reconnection processes in the interplanetary
magnetic field, as already found by Bavassano et al. (1976) and McComas et al. (1994) in the topology of the magnetic field, can be associated with
proton and -particle bursts in the proton energy range 6.5 to 100 keV/e.
According to Bavassano et al. (1976), the resistive tearing mode instability
is responsible for the reconnection process.
The SWICS measurements (E/q=0.5-31.5 keV/e) demonstrated that often a
broader distribution of low energy ions precedes the >6.5 keV/e burst,
probably caused by the accelerating force of reconnected field lines
(compare Zurbuchen et al. 2001, Fig. 10).
The particle energy resulting from the X-line formation process can be
calculated using the equation:
(see, e.g. Ip & Axford 1986),
|where||=||is the Alfvén velocity|
|B||=||magnetic field magnitude|
|L||=||length of the neutral line|
With typical values for
ions/cm3 and B=6 nT, one finds for the
km s-1 and for the proton energy 100 keV when
the length of the neutral line
km is assumed, which seems not to
be unreasonable for the interplanetary space.
Fitzenreiter & Burlaga (1978) already found that magnetic holes
can have an extension >105 km. The particle and magnetic
field measurements in the libration point L1 and near the Earth let us
conclude that the magnetic field can maintain its shape for 1 h during
which sporadic particle acceleration takes place.
Farrugia et al. (2001) described a reconnection event associated with a
magnetic cloud which was also observed by WIND on 24 Dec. 1996. They
interpreted this observation as MHD discontinuity arriving from a
reconnection side closer to the Sun. We analysed the same event according
to our method (not shown as figure)
and found that the plasma measured by SWICS (0.5-31.5 keV/e)
was accelerated, as well as a short-lived burst of 6.5-225 keV/e
ions. The maximum velocity reached by the ions was 9 times the solar
wind velocity. The sector measurements of the STICS sensor indicate
that the acceleration of the suprathermal ions started most likely
somewhat antisunward of the WIND-S/C. The suprathermal ion bursts shown
in our Figs. 1-6 and the example described by Farrugia (24 Dec. 1996)
are most likely accelerated locally and not close to the Sun, otherwise
the 6.5 keV/e and the 100 keV/e ions should show a distinct velocity
dispersion. The travel time of 100 keV/e and 6.5 keV/e protons over
a distance 1 AU is 9.5 h and 37 h, respectively.
However, we cannot exclude that near the interplanetary current sheet a
still unkown mechanism exists which can also accelerate protons to 100 keV.
Burlaga & Lamaire (1978) predict in their theory of magnetic holes
an electric field in z-direction of
V/m at the beginning
and at the end of the magnetic field depression. Such an electric
field would require an acceleration length of 105 V/10-5
km which seems to be unrealistically large.
Figures 1 and 3, respectively, Figs. 4 and 5 showed that suprathermal
bursts appeared at the beginning or at the end of the magnetic field
depression. Therefore, we check whether an inductive electric field
could be responsible for the particle acceleration.
This paper is dedicated to our colleague Dr. Berend Wilken who passed away on 4 September 2001. His contribution to the Time of Flight Technology applied in the SMS experiment was important for its success. The authors thank Drs. R. P. Lepping and M. Acuna for magnetic field and solar wind measurements used in this study and the German BMFT for financial support. We also thank Drs N. Ness, D. J. McComas, R. Gold for magnetic field, solar wind and ion measurements of the ACE-S/C and A. Szabo, R. P. Lepping for Magnetic field measurements of IMP8 which we received via CDAWeb.