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Waves in the ultra‐low‐frequency (ULF) band have frequencies which can be drift resonant with electrons in the outer radiation belt, suggesting the potential for strong interactions and enhanced radial diffusion. Previous radial diffusion coefficient models such as those presented by Brautigam and Albert (2000) have typically used semiempirical representations for both the ULF wave's electric and magnetic field power spectral densities (PSD) in space in the magnetic equatorial plane. In contrast, here we use ground‐ and space‐based observations of ULF wave power to characterize the electric and magnetic diffusion coefficients. Expressions for the electric field power spectral densities are derived from ground‐based magnetometer measurements of the magnetic field PSD, and in situ AMPTE and GOES spacecraft measurements are used to derive expressions for the compressional magnetic field PSD as functions of Kp, solar wind speed, and L‐shell. Magnetic PSD results measured on the ground are mapped along the field line to give the electric field PSD in the equatorial plane assuming a guided Alfvén wave solution and a thin sheet ionosphere. The ULF wave PSDs are then used to derive a set of new ULF‐wave driven diffusion coefficients. These new diffusion coefficients are compared to estimates of the electric and magnetic field diffusion coefficients made by Brautigam and Albert (2000) and Brautigam et al. (2005). Significantly, our results, derived explicitly from ULF wave observations, indicate that electric field diffusion is much more important than magnetic field diffusion in the transport and energization of the radiation belt electrons. Key Points Ground magnetometers can be used to estimate the E‐field diffusion coefficients Previous models overestimate the magnetic diffusion coefficients Electric field diffusion can be more important than magnetic diffusion

GOES‐16 and GOES‐17 are the first of NOAA's Geostationary Operational Environmental Satellite (GOES)‐R series of satellites. Each GOES‐R satellite has a magnetometer mounted on the end (outboard) and one part‐way down a long boom (inboard). This paper demonstrates the relative accuracy and stability of the measurements on a daily and long‐term basis. The GOES‐16 and GOES‐17 magnetic field observations from 2017 to 2020 have been compared to simultaneous magnetic field observations from each other and from the previous GOES‐NOP series satellites (GOES‐13, GOES‐14 and GOES‐15). These comparisons provide assessments of relative accuracy and stability. We use a field model to facilitate the inter‐satellite comparisons at different longitudes. GOES‐16 inboard and outboard magnetometers data suffer daily variations which cannot be explained by natural phenomena. Long‐term‐averaged GOES‐16 outboard (OB) data has daily variations of ±3 nT from average values with one‐sigma uncertainty of ±1.5 nT. Long‐term averaged GOES‐17OB magnetometer data have minimal daily variations. Daily average of the difference between the GOES‐16 outboard or GOES‐17 outboard measurements and the measurements made by another GOES satellite are computed. The long‐term averaged results show the GOES‐16OB and GOES‐17OB measurements have long‐term stability (±2 nT or less) and match measurements from magnetometers on other GOES within limits stated herein. The GOES‐17OB operational offset (zero field value) was refined using the GOES‐17 satellite rotated 180° about the Earth pointing axis (known as a yaw flip).

We have studied the solar cycle variation of equatorial plasma mass density ρeq in the plasma trough at geosynchronous altitude. The density was indirectly determined from the frequency, fT3, of the third harmonic of toroidal standing Alfvén waves detected over a 12 year period from 1980 to 1991 with magnetometers on five Geostationary Operational Environmental Satellites (GOES). Realistic models of the ambient magnetic field and field line mass distribution were used in numerically solving the wave equation to relate fT3 to ρeq. Scanning the magnetometer data in a 30 min time window that moved forward in 10 min steps, we obtained 228,382 fT3 samples equivalent to 1586 days of data. The detection rate of fT3 is highest (∼50%) in the prenoon sector, and fT3 and ρeq samples from this sector were used to examine their dependence on F10.7, Kp, and Dst. Overall, F10.7 exhibits the highest correlation with fT3 and ρeq, implying that the solar UV/EUV control of ion production at the ionospheric height is strongly reflected in mass density variations at geosynchronous orbit. Using 27 day medians computed excluding periods of plasmasphere expansion to geosynchronous orbit and geomagnetic storm, we obtained the empirical formula fT3 (mHz) = 38 − 0.097F10.7 and logρeq (amu cm−3) = 0.42 + 0.0039F10.7, where F10.7 is given in the solar flux units 10−22 W · m−2 · Hz−1. This last formula means that with the 27 day F10.7 in the range of 68–255 in the selected solar cycle, the mass density varied by a factor of ∼5 from ∼5 to ∼26 amu cm−3. During extremely quiet times (Kp averaged using a 3 day time scale <1), for which the plasmasphere may extend out to geosynchronous orbit, and during storm periods (Dst < −50 nT), the mass density may be enhanced beyond these values.

We examine the global field‐aligned current (FAC) topology associated with a clear substorm on the 16 February 2010. We show that for this particular substorm there is a clear and localised reduction in the FACs observed by AMPERE at least 6 minutes prior to auroral onset. A new auroral arc forms in the region of reduced FAC and on closed field lines which subsequently brightens and expands poleward, signifying the start of the substorm expansion phase. We argue that the change in FACs observed prior to onset is the result of a change in the magnetosphere‐ionosphere (M‐I) coupling in a region local to the subsequent auroral onset. Such a change implies an important role for M‐I coupling in destabilising the near‐Earth tail during magnetospheric substorms and perhaps more importantly in selecting the location in the ionosphere where auroral onset begins. Key Points Localised reduction FACs is observed preceding and local to auroral onset Change in FACs is the result of a change in magnetosphere‐ionosphere coupling These observations have important implications in any substorm paradigm

Despite the characterization of the auroral substorm more than 40 years ago, controversy still surrounds the processes triggering substorm onset initiation. That stretching of the Earth's magnetotail following the addition of new nightside magnetic flux from dayside reconnection powers the substorm is well understood; the trigger for explosive energy release at substorm expansion phase onset is not. Using ground‐based data sets with unprecedented combined spatial and temporal coverage, we report the discovery of new localized and contemporaneous magnetic wave and small azimuthal scale auroral signature of substorm onset. These local auroral arc undulations and magnetic field signatures rapidly evolve on second time scales for several minutes in advance of the release of the auroral surge. We also present evidence from a conjugate geosynchronous satellite of the concurrent magnetic onset in space as the onset of magnetic pulsations in the ionosphere, to within technique error. Throughout this time period, the more poleward arcs that correspond to the auroral oval which maps to the central plasma sheet remain undisturbed. There is good evidence that flows from the midtail crossing the plasma sheet can generate north‐south auroral structures, yet no such auroral forms are seen in this event. Our observations present a severe challenge to the standard hypothesis that magnetic reconnection in stretched magnetotail fields triggers onset, indicating substorm expansion phase initiation occurs on field lines that are close to the Earth, as bounded by observations at geosynchronous orbit and in the conjugate ionosphere.

To provide critical ULF wave field information for radial diffusion studies in the radiation belts, we quantify ULF wave power (f = 0.5–8.3 mHz) in GOES observations and magnetic field predictions from a global magnetospheric model. A statistical study of 9 years of GOES data reveals the wave local time distribution and power at geosynchronous orbit in field‐aligned coordinates as functions of wave frequency, solar wind conditions (Vx, ΔPd and IMF Bz) and geomagnetic activity levels (Kp, Dst and AE). ULF wave power grows monotonically with increasing solar wind Vx, dynamic pressure variations ΔPd and geomagnetic indices in a highly correlated way. During intervals of northward and southward IMF Bz, wave activity concentrates on the dayside and nightside sectors, respectively, due to different wave generation mechanisms in primarily open and closed magnetospheric configurations. Since global magnetospheric models have recently been used to trace particles in radiation belt studies, it is important to quantify the wave predictions of these models at frequencies relevant to electron dynamics (mHz range). Using 27 days of real interplanetary conditions as model inputs, we examine the ULF wave predictions modeled by the Lyon‐Fedder‐Mobarry magnetohydrodynamic code. The LFM code does well at reproducing, in a statistical sense, the ULF waves observed by GOES. This suggests that the LFM code is capable of modeling variability in the magnetosphere on ULF time scales during typical conditions. The code provides a long‐missing wave field model needed to quantify the interaction of radiation belt electrons with realistic, global ULF waves throughout the inner magnetosphere.

Multipoint observations of a dayside Pc4 pulsation event provide evidence of fast mode waves trapped in the plasmasphere (plasmaspheric cavity mode or virtual resonance). Time History of Events and Macroscale Interactions during Substorms (THEMIS)‐A, the primary source of data for the present study, was moving outward near noon and detected poloidal oscillations, characterized by the azimuthal electric field component Ey and the radial and compressional magnetic field components Bx and Bz. The structure of the plasmasphere was constructed from the mass density radial profile estimated from the frequency of toroidal standing Alfvén waves observed at this spacecraft. The outer edge of the plasmapause (the maximum of the equatorial Alfvén velocity VAeq) was located at L ∼ 7, and the minimum of VAeq was located at L ∼ 4, forming a potential well structure required for mode trapping. Relative to the ground magnetic pulsations observed in the H component at a low‐latitude station (L = 1.5), the Ey component exhibited a broad amplitude maximum around L ∼ 3.5 and maintained a nearly constant phase from L = 2 to L = 5. In contrast, the Bz component exhibited an amplitude minimum and switched its phase by 180° at L = 3.8. This radial mode structure is consistent with theoretical models of mode trapping. Also, the Ey and Bz components oscillated ±90° out of phase, as is expected for radially standing waves.

The Combined Release and Radiation Effects Satellite (CRRES) mission provides an opportunity to study the distribution of MHD wave power in the inner magnetosphere both inside the high‐density plasmasphere and in the low‐density trough. We present a statistical survey of Pc5 power using CRRES magnetic field, electric field, and plasma wave data separated into plasmasphere and trough intervals. Using a database of plasmapause crossings, we examined differences in power spectral density between the plasmasphere and trough regions. These differences were typically a factor of 3 or 4 but could be as much as an order of magnitude and could be seen in both electric and magnetic field data. Our study shows that determining the plasmapause location is important for understanding and modeling the MHD wave environment in the Pc5 frequency band.

On day 108, 2001, the Sub‐Auroral Magnetometer Network (SAMNET) and Magnetometers along the Eastern Atlantic Seaboard for Undergraduate Research and Education (MEASURE) magnetometer arrays detected dayside magnetic pulsations at a common frequency of ∼15 mHz at all locations below L = 4. This global pulsation event was associated with alignment of the interplanetary magnetic field with the Sun‐Earth axis, a condition known to generate ultralow‐frequency (ULF) waves in front of the bow shock. The event occurred during the early recovery phase of a geomagnetic storm. Magnetic field measured by the GOES 8 geostationary satellite on the dayside indicated elevated broadband (7–80 mHz) ULF power in the compressional component without a strong peak at 15 mHz. These observations suggest that the global pulsations originated from a compressional magnetohydrodynamic eigenmode oscillation of the inner magnetosphere stimulated by a broadband external disturbance. The equatorial Alfvén velocity corresponding to the toroidal frequencies that were determined with the cross‐phase analysis of SAMNET and MEASURE data showed a gradual decrease of the velocity with L without a clear signature of a plasmapause. The observed properties of the global pulsations are consistent with virtual resonance in the inner magnetosphere.

The relationship between solar wind dynamic pressure changes and geosynchronous magnetic field response is studied using 15 years of OMNI2 and GOES data at 1‐minute resolution. Significant magnetospheric response to solar wind‐forcing is found to be most frequent near noon (30% of all intervals), and virtually absent on the night‐side. The strongest response occurs when IMF Bz is strongly northward and the effect of reducing IMF Bz is most pronounced in the dusk sector. Approximately 25% of dayside Bz variance for related intervals can be attributed to direct response from solar wind dynamic pressure forcing. Time lag between changes in the solar wind at the bow shock nose and similar fluctuations in the magnetosphere at geosynchronous orbit (6.6 Re) is typically 2 to 4 minutes, with responses occurring first in the post‐noon sector and approximately 2 minutes later near dawn. The OMNI2 HRO time‐shifting algorithm appears to be quite effective, with a slight (2 minute) systematic increase in lag and no increased scatter for the most distant upstream solar wind satellite location. Key Points Ubiquitous geostationary magnetic field response to solar wind pressure forcing Time delay of 2‐4 min from bow shock impact, first at post‐noon sector OMNI2 time‐shifting algorithm is very effective