W. Magnes

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Correlation of substorm injections, auroral modulations, and ground Pi2

[1]xa0In this case study we report a substorm, 23 March 2007, which exhibited oscillations with a period of ∼135 s in three substorm phenomena all of which were one-to-one correlated. The in-situ observations are from one THEMIS spacecraft (8.3 RE geocentric distance) and the geosynchronous LANL-97A spacecraft. The focus here is on the intensification phase during which THEMIS was conjugate to the region of auroral brightening and its foot point was near the high-latitude ground station Kiana. The following results will be demonstrated: (1) THEMIS and LANL-97A (time-delayed) recorded periodic ion injections (>100 keV). (2) Near-conjugate high-latitude ground magnetometer data show very large Pi2 (δH∼150 nT) with a 6-s time delay compared to the THEMIS ion injections. (3) Low-latitude ground magnetometer data also show Pi2 with the same waveform as the high-latitude Pi2 but with longer time delays (20–31 s). (4) Auroral luminosity was periodically modulated during the intensification phase. (5) All three signatures (ion injections, ground Pi2, optical modulation) had the same periodicity of ∼135 s but with various time delays with respect to the THEMIS ion injections. These observations demonstrate that the three substorm phenomena had a common source which controlled the periodicity.

Multiple intensifications inside the auroral bulge and their association with plasma sheet activities

[1]xa0In this coordinated ground and space study, we report multiprobe measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS), LANL-97A, Polar, and ground observatories for a substorm that occurred on 23 March 2007. The THEMIS fleet and LANL-97A were located in the premidnight, near-Earth plasma sheet in the radial range from 6.6 to 13 RE, placing the spacecraft into different plasma environments which were subject to different activities. Simultaneous global Polar Ultraviolet Imager images of the aurora revealed a fine structure in the auroral bulge in the form of several time-delayed regions of brightening. We demonstrate a correspondence between this fine structure and the spatially separated plasma sheet activities (substorm injections with energies >100 keV) by showing that both executed periodic (100–150 s) one-to-one correlated modulations. Additionally, the different auroral brightening regions were modulated approximately out of phase to one another, as were the separated plasma sheet activities. The periodic plasma sheet and optical modulations were also one-to-one correlated with large-amplitude (δH ∼ 150 nT) ground Pi2 pulsations. In contrast to the most energetic ions (>100 keV), the lower-energetic plasma sheet ions executed separate oscillations during the development of the substorm, including the preintensification phase, and showed the following properties. (1) The oscillation periods were different at different spacecraft locations and had a tendency to increase during the evolution of the substorm. During the preintensification phase, multiple (possibly harmonic) spectral components existed. (2) The oscillations were coupled to westward moving perturbations of an energized plasma boundary. The boundary perturbations were likely conjugate to azimuthally spaced auroral forms (“beads”) observed by Polar-UVI during the preintensification phase and could play a role in the onset of the substorm intensification. (3) The oscillations of the lower-energetic ions were also one-to-one correlated with smaller-amplitude ground Pi2 pulsations (<15 nT). In conclusion, the combination of these observations allowed us to construct a 3-D picture of low-frequency, near-Earth plasma sheet phenomena associated with a substorm and their connection to aurora and the ground. It appeared that not only one substorm current wedge, but additional current structures existed which started at different times, pulsated out of phase, and mapped from different active regions into the ionosphere. The active space regions appeared to be coupled and transferring energy from one region to the other while pulsating. We propose that the wave-like structures in the plasma sheet, observed before and during the substorm/intensification phase, and their demonstrated properties support a wave phenomenon (such as a ballooning-type mode) for the onset and development of the substorm/intensification, rather than directly driven periodic bursty bulk flow activations.

Solar activity dependence of geomagnetic field line resonance frequencies at low latitudes

[1] ULF field line resonance frequencies (f R ) of three different magnetic shells (L = 1.61, 1.71, and 1.83) have been monitored during a 4-year period (2001-2004) using a cross-phase analysis of magnetic measurements recorded at the South European Geomagnetic Array (SEGMA). We find that the variations of the daily averages of f R , which reflect changes in plasmaspheric mass density, follow the variations of the daily values of the 10.7-cm solar radio flux F 10.7 with an estimated time delay of 1-2 days. The analysis of selected events indicates that the sensitivity of f R to short-term (27-day) variations in the solar flux is the same as that for long-term (solar cycle related) variations. On the other hand, the results of the overall statistical analysis seem to indicate a lower sensitivity (by a factor of ∼2) of f R to short-term solar flux variations with respect to long-term variations. Geomagnetic activity effects and/or different solar variabilities of EUV and 10.7-cm flux for different timescales are suggested as a possible cause for such a difference. Experimental results are also compared with those provided by a physical-numerical model of the ionosphere-plasmasphere system. Last, we find some evidence for a slight annual variation in f R with an estimated summer/winter ratio of ∼ 1.1-1.2. The inferred corresponding annual variation in the equatorial mass density is in line with previous estimates for the European longitudinal sector as obtained from whistler measurements.

Highly periodic stormtime activations observed by THEMIS prior to substorm onset

[1]xa0On March 24, 2007 THEMIS observed near the dusk flank several 10 minute quasi-periodic flow and magnetic field oscillations followed by the onset of a strong substorm (AL ∼ −1000 nT). The substorm occurred during an interval of strongly southward IMF, near the start of the recovery phase of a small storm (SYM-H near −80 nT). Each magnetic oscillation was accompanied by a rapid flow variation, auroral intensification, energetic particle injection, and Pi2 pulsations. For several hours both prior to and following the substorm THEMIS observed highly periodic flow oscillations, with the same 10 minute periodicity. The average of these flow oscillations was non-zero and positive, indicating net sunward transport. We suggest that the long interval of oscillatory flow constituted a periodic convective mode of the magnetosphere, and further suggest that the quasi-periodic activations were associated with reconnection near the THEMIS location.

ULF Wave Magnetic Measurements by CHAMP Satellite and SEGMA Ground Magnetometer Array: Case Study of July 6, 2002

We present the analysis of a Pc 3 geomagnetic pulsation event observed simultaneously by CHAMP and by the South European GeoMagnetic Array SEGMA (1.56 < L < 1.88) during the conjunction of July 6, 2002. Both compressional and transverse oscillations were identified in CHAMP magnetic measurements. A close correspondence between the compressional component and the ground signals is observed. At the same time the joint analysis of space and ground observations clearly indicates the occurrence of a field line resonance at L ✠ 1.6. A direct confirmation of the well known 90° rotation of the ULF wave polarization ellipse through the ionosphere is also provided.