Hiroshi Matsui

Cold dense plasma in the outer magnetosphere

Cold dense plasma (CDP) with energies less than 32 eV in the outer magnetosphere, L ∼ 10, is reported using data obtained by Geotail. We have found unexpectedly recurrent CDP in afternoon magnetic local times. CDP is often accompanied by westward flows, indicating that the regions with CDP are located outside the corotation region and the plasma is presumably convected toward the magnetopause. CDP is observed in the cases of both small and moderate geomagnetic activity. We have calculated the location of the stagnation point as a function of cross-tail potential by using a Volland-Stern-type [Volland, 1973; Stern, 1975] convection model. Stagnation point locations are variable even if the cross-tail potential or geomagnetic activity is small, which could lead to a high rate of the observation of plasmaspheric material at the Geotail orbit. However, part of the cold plasma is directly supplied by the ionosphere without passing through the plasmasphere. The number of supplied particles could be larger on the drift orbits close to the stagnation points. Finally, an average quantity of cold plasma flowing through the Geotail orbit with quasi-stable westward convection is estimated to be 3.6 × 10 25 ions/s with a standard deviation of 3.7 × 10 25 ions/s, as compared to 9.4 - 14 × 10 25 ions/s for the polar wind toward the plasma sheet.

Wind and ACE observations during the great flow of 1-4 May 1998: Relation to solar activity and implications for the magnetosphere

[1]xa0Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME-driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3-hour burst of high-speed flow on 4 May. A statistical survey using the OMNI database for the 6-year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.

Electric field measurements in the inner magnetosphere by Cluster EDI

[1]xa0We report electric field measurements in the inner magnetosphere from the Electron Drift Instrument (EDI) on Cluster at distances of 4 0) and 2.45 (BZ < 0). These electric fields are discussed in terms of their sources such as the ionospheric dynamo and the solar wind-magnetosphere interaction. We also quantify the electric field as follows: the average shielding parameter γ [Volland, 1973; Stern, 1975] is ∼2, although the value for southward IMF is smaller than the value for northward IMF. The parameter γ is larger on the duskside than in the dawn sector. The average rate of sunward transport of magnetic flux, equivalent to the duskward electric field, is estimated as 0.32 mV/m for southward IMF. Finally, fluctuations tend to be larger than the DC component around the stagnation point, which could lead to outflow of plasmaspheric material.

Interplanetary coronal mass ejection and ambient interplanetary magnetic field correlations during the Sun‐Earth connection events of October–November 2003

[1]xa0Magnetic field observations made during 28 October to 1 November 2003, which included two fast interplanetary coronal mass ejections (ICMEs), allow a study of correlation lengths of magnetic field parameters for two types of interplanetary (IP) structures: ICMEs and ambient solar wind. Further, they permit the extension of such investigations to the magnetosheath and to a distance along the Sun-Earth line (X) of about 400 RE. Data acquired by three spacecraft are examined: ACE, in orbit around the L1 point; Geotail, traveling eastward in the near-Earth solar wind (at R ∼ 30 RE); and Wind, nominally in the distant geomagnetic tail (R ∼ −160 RE) but making repeated excursions into the magnetosheath/solar wind due to the flapping of the tail. Analyses are presented in both time and frequency domains. We find significant differences in the cross-correlation/coherence properties of the ambient interplanetary magnetic field (IMF) and ICME parameters. For the ambient IMF, we find high coherence to be confined to low frequencies, consistent with other studies. In contrast, ICME magnetic field parameters remain generally coherent up to much higher frequencies. Scale lengths of ICME magnetic field parameters are in excess of 400 RE. High speeds of ∼1700 km s−1 are inferred from the plot of phase difference versus frequency, consistent with that obtained from plasma instruments. To strengthen these results and to extend them to include dependence on the distance perpendicular to the Sun-Earth line (Y), we examine a 28-day interval in year 2001 characterized by a sequence of 10 ICMEs and containing roughly equal ambient solar wind and ICME time intervals. ACE-Wind X and Y separations were ∼220 and ∼250 RE, respectively. We find good coherence/correlation alternating with poor values. In particular, we find that in general ICME coherence/correlation lengths along Y are larger by a factor of 3–5 than those quoted in the literature for ambient solar wind parameters. Our findings are good news for the space weather effort, which depends crucially on predicting the arrival of large events, since they make possible the placement of upstream monitors to give a longer lead time than at L1.

Power to the magnetosphere: May 4, 1998

An extraordinary powering of the magnetosphere by the solar wind occurred in a 3-hour burst early on May 4 when the IMF was very intense and pointed south (≈-35 nT; “erosion phase”). Examining solar wind streams over 3 months, we found that May 4 represented a very fast, hot, non-corotating stream overtaking an interplanetary coronal mass ejection (ICME), thus forming a compound stream. By integrating the “epsilon” parameter over time, we find that the energy deposited in the magnetosphere during the erosion phase on May 4 (of order 7.5 J m−2) was higher to that deposited during the previous 3-day period, itself a very geoeffective interval. We compare the energy and power supply to the magnetosphere on May 4 with 13 other events, mainly ICMEs and magnetic clouds, during the period 1995–2000. Specifically, we examine (a) the total energy input over 3 days, and (b) the average power over a 3-hour period near maximum power of the respective configurations. As regards (a), we find the energy of the May 4 stream to be comparable to that of the strong events observed during the 6-year period. As regards (b), we find May 4 to represent a large fluctuation from the norm, exceeded only by the Bastille Day event (July 15, 2000). The ability to predict a concentration of electromagnetic power and energy such as that in the May 4 fast stream poses a challenge to our ability to predict space weather.

Outflow of cold dense plasma associated with variation of convection in the outer magnetosphere

Abstract Cold dense plasma with the ionospheric origin is often observed in the outer magnetosphere with L values as large as 10. We have examined the electric field data accompanied by the cold dense plasma. The electric field data are obtained by the direct measurement of the drift motion of electrons released from electron guns. We get westward components of convection. In addition, there is an AC component of electric field in the ULF range larger than the DC component. If such a large variation of electric field exists in the vicinity of the stagnation point, the plasmaspheric plasma is expected to flow away to the outer magnetosphere. Then we have a test particle simulation by adding a potential variable in time. There is a particle outflow accompanied by the AC variation of electric field in the ULF range. If there are more complicated variations of electric field, it is possible that particles flow out frequently.

Coherence Lengths of the Interplanetary Electric Field: Solar Cycle Maximum Conditions

It is increasingly being realized that by affecting geoeffective scale lengths the interplanetary electric field (IEF) is a key quantity in space weather discussions. In this work we derive and analyze statistically IEF coherence lengths in year 2000, i.e., near maximum of solar cycle 23, working in a much used formulation for the IEF. We focus on the frequency domain. We use magnetic field and plasma data sets acquired by Wind and ACE. During year 2000, ACE‐Wind separations were very variable and, in particular, Wind’s first dayside distant prograde orbit resulted in a Y‐separation comparable to the X‐separation (∼220 RE). We find IEF coherence lengths of 200–250 RE (X) and 50–100 RE (Y). The coherence is mainly carried by the low frequency components (f < 0.01 min−1).