Harri Laakso

During fast mode intervals the Magnetospheric Multi‐Scale (MMS) Fast Plasma Investigation (FPI) operates continuously in ion burst capacity at 150 msec cadence. From the burst data and at the same cadence, FPI computes four ion moments (ion density, ion flux) termed trigger data. Only a few percent of the full burst data are downloaded, but trigger data are downloaded over the entire fast mode interval. The trigger data were not envisioned as a science product, so the sign bits of the 3 ion flux components were dropped, and only absolute values were telemetered to the ground. Using 4.5 s FAST mode data and minimization techniques, we reconstruct the ion flux sign bits and computed ion velocity, validating the methodology through comparisons with burst mode velocity for 103 burst mode intervals. We demonstrate that calibrated FPI “trigger” data are scientifically valid in the magnetosheath and its boundary layer, and particularly useful for wave studies whose frequency is greater than 0.1 and less than 3 Hz. As an example of the use of the trigger data, we present an analysis of ion cyclotron waves (ICW) observed by the MMS constellation on 7 October 2015, in the afternoon magnetosheath.

On 10 May 2024, Earth was hit by a CME that drove the largest geomagnetic storm in 20 years. Multi‐spacecraft observations previously showed that the ∼100 nT north‐south IMF variation was driven by Kelvin‐Helmholtz waves with wavelength ∼250 and reconnection jets in the ‐direction (Nykyri, 2024a, https://doi.org/10.1029/2024gl110477 ). Here we show that Magnetospheric Multiscale (MMS), located upstream of the Earth's bow shock at , detected additional reconnection jets not seen by ARTEMIS at lunar orbit ∼60 away, perpendicular to CME propagation. One jet coincided with high‐energy ions and electrons streaming anti‐parallel to the magnetic field. The ions showed dispersion followed by apparent counter‐dispersion, whereas electrons above 100 keV arrived nearly simultaneously. Strong electromagnetic wave activity between these phases is consistent with magnetosonic‐Whistler waves, suggesting that MMS traversed very near the particle acceleration region.

We investigate the empirical relationship between the spacecraft potential ( V _s ) measured by the Electric Field Double Probes, and the electron density ( N _e ) measured by the Fast Plasma Instrument on the MMS spacecraft. We derive their relationship during fast‐mode intervals when the Active Spacecraft Potential Control Devices are off. Then we apply this relationship to slow‐mode intervals during the perigee passes where V _s can be less than +2 V and N _e can exceed 1,000 cm ⁻³ . Because such a parameter range is never observed by MMS during fast‐mode intervals, we define this part of the relationship using simultaneous observations from the Van Allen mission where N _e is measured up to 3,000 cm ⁻³ while V _s is less than +2 V. We compare the empirical relationship to the predictions by an orbital motion limited theory. This suggests how to model the photoelectron current above +5 V and the collection of ambient electrons for moderate Debye lengths. We apply the empirical relationship (i.e., theoretical curves are not used here) to several consecutive plasmapause crossings by MMS during two magnetic storms. The erosion rates of the plasmasphere were ∼60 cm ⁻³ /day at 7 MLT and ∼30 cm ⁻³ /day at 14 MLT. In the duskside, the erosion rate decreases with L due to increasing flux tube volumes, being about 10–20 cm ⁻³ /day at L ∼ 7. The refilling rates are similar to the erosion rates, but in the dawnside, the refilling is delayed due to a slow expansion of the plasmapause to higher L .

Our study presents the first simultaneous multi‐point observations of the local‐time asymmetry of 10's–100's of keV energy particle pitch angle distributions (PADs), and associated wave activity during the main phase of the geomagnetic storm that occurred on 7 September 2017. This study is a companion to Pandya et al. (2025), who identified a rare conjunction in which RBSP‐A and RBSP‐B were located in the evening and morning sectors, respectively, enabling the comparison of ion populations across local time. Building on that work, we identify an additional rare conjunction during the same interval in which RBSP‐A and MMS‐1 were co‐located in the evening sector for ∼15 min (2,325–2340 UT) within Δ L < 0.2, ΔMLT < 2 min and ΔMLAT ∼ 20°. Our analysis shows strong local time asymmetries. Protons in the evening sector exhibited freshly injected, enhanced fluxes with nearly isotropic PADs at >60 keV energies and pancake PADs at <60 keV energies, whereas dawn‐noon proton fluxes retained pancake PADs characteristic of older populations. In contrast, electrons showed a significant flux decrease in the evening sector and isotropic enhancement in the morning sector, consistent with their eastward gradient‐B and curvature drifts. Concurrent wave observations revealed strong ULF, magnetosonic and hiss waves in the evening sector, while hiss waves dominated in the morning sector. Our results demonstrate that storm‐time magnetic field reconfiguration, rather than wave‐particle interactions, primarily governs keV particle PAD asymmetries. The findings highlight the importance of coordinated multi‐point measurements for advancing our understanding of ring current asymmetries and storm‐time particle transport.

On May 10th, 2024, Earth was hit by a CME, creating the largest geomagnetic storm in 20-years. The up to ~100 nT north-south bz variation of the interplanetary magnetic field (IMF) was driven by Kelvin-Helmholtz waves with ~250 RE in wavelength, including reconnection jets in +- z-direction (Nykyri, 2024). Here, we show, that Magnetosphere Multiscale (MMS) spacecraft that also detected additional reconnection jets, not seen by ARTEMIS at lunar orbit ~50 RE away, perpendicular to CME propagation. One of the reconnection bursts was associated with high-energy ion and electron fluxes anti-parallel to the magnetic field. The ions showed energy-time-dispersion, followed by counter-dispersion, whereas electrons, due to their relativistic speed, appeared nearly simultaneously at all energy channels above 100 keV. Between the dispersion and counter-dispersion signature appeared strong electromagnetic wave activity, consistent with magnetosonic-Whistler waves, hinting that MMS may have flown near the particle acceleration region.

We investigate the fastest ExB plasma drifts observed by the Vector Electric Field Instrument double probe experiment on the NASA's Dynamics Explorer‐2 (DE‐2) satellite. In the limited DE‐2 database, we find 507 events where the zonal ExB velocity exceeds 4 km/s, in which for 91 events the velocity exceeds 6 km/s and 16 events where the drifts are greater than 8 km/s. One primary group of fast drifts consists of nightside events that occur mostly within the shadowed ionosphere and are related to auroral precipitation (e.g., inverted‐V, sub‐auroral ion drifts, Alfven waves) driven by magnetospheric processes. Another major group of fast drifts corresponds to dayside events that are often directly driven by the solar wind and can occur within either the sunlit or shadowed ionosphere. The fast drifts are observed at all DE‐2 altitudes between 300 and 1,000 km with no altitude dependence on the drift velocity. They are generally observed at all MLT sectors although there appears to be a dawn‐dusk asymmetry with the fewest events around 15 MLT. In contrast, the occurrence peaks between 6 and 12 MLT. The events occur in the auroral and sub‐auroral altitudes, normally between 60 and 83° of invariant latitude, although in the pre‐midnight sector, events are found as low as 50° of invariant latitude, corresponding to sub‐auroral ion drift (SAID) events. Surprisingly, with the exception of SAID events, the fast drift events occur during all geomagnetic conditions with the velocities showing no dependence on geomagnetic activity.

Whitepaper #321 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; basic research. Additional topics: space weather research/applications/operations pipeline; planetary ionospheres / upper atmospheres; […]

Whitepaper #322 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: basic research. Additional topics: planetary ionospheres / upper atmospheres.

We have identified over 200 fast plasma drift events (>5 km/s) in the high‐latitude ionosphere, as measured by the electric field double probe on the Dynamics Explorer‐2 satellite during its 18 months lifetime from August 1981 to February 1983. Although these drifts can appear at all local time sectors, they are most common on the dawnside and in the pre‐midnight sector. The fast plasma drifts generally occur between 64° and 82° inv.lat. although they sometimes appear below 55° inv.lat. in the pre‐midnight sector, in which case they appear associated with a phenomenon often referred to as sub‐auroral ion drifts. For increasing Auroral Electrojet index, their occurrence rate increases and their latitudinal locations move equatorward. The observed events last between 0.1 and 60 s, corresponding to (north‐south) widths of 1–500 km along the satellite trajectory. The observations show a seasonal dependence such that the faster drifts tend to occur over the dark (winter) hemisphere.

On the 18th of August 2002, during a crossing of the near‐Earth plasma sheet Cluster observed an ion flow burst, caused by a near‐Earth reconnection event. Cluster observed a tailward bulk flow which reverse to earthward flow, indicating a close passage of the diffusion region. We show that reversals in B_Z and B_Y are consistent with reconnection. During the event, a short duration burst of electrons in the range of a few keV up to more than 100 keV are observed streaming away from the reconnection region. The accelerated electrons were aligned with the magnetic field, streaming tailward, and were observed simultaneously by all four spacecraft located on both the northern and southern side of the current sheet. The four Cluster spacecraft, separated by ∼3700 km, observe the electrons for a time period of ∼60 s, indicating the burst to be a temporal rather than localized feature. A second burst of tailward accelerated electrons observed for ∼40 s was observed by Cluster 1 and 2 upon entering the earthward outflow region. The second beam thus appear to be directed toward the X‐line. The flux levels of the energetic electron bursts exceed those of the ambient plasma sheet by a factor 2–4. In general, the highest energetic electron fluxes during this event were observed in the earthward outflow region. Observations indicate that reconnection operates on closed plasma sheet field lines in this event and does not reach lobe field lines.