An ultralow frequency (ULF) wave was simultaneously observed in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) radar at Hankasalmi, Finland and on the ground by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers with close proximity to the radar. The onset time of the wave event was around 03:00 magnetic local time. Fourier wave analysis of the event suggests a wave period of about 1,340 s with an equatorward latitudinal and eastward longitudinal wave phase propagation, and an effective azimuthal wave number of 17 +/- 1, in the intermediate range of those observed in ULF waves. This wave has been interpreted as resulting from drifting electrons of energies of 13 +/- 5 keV in a drift resonance condition linked to energetic particle populations during a magnetospheric substorm. The latitudinal phase characteristics of this wave experienced temporal evolution, believed to be caused by additional injected particle populations associated with the same substorm driving the wave, which resulted in an observed loss of HF backscatter. This observation of a unique type of temporal evolution in the phase propagation characteristics of ULF waves enhances current understanding about the structure, dynamics and source of these types of ULF waves.

We have modeled the individual harmonic frequencies of the Ionospheric Alfven Resonator (IAR) at Eskdalemuir by solving a one-dimensional wave equation and using non-uniform modeled Alfven velocity profiles. By comparing the results of the modeling alongside harmonics obtained from the Eskdalemuir, UK, data set from 2013 to 2021, the effects of the non-uniformity of the Alfven velocity profile on the IAR are considered. We calculated the offset between the fundamental frequency and the harmonic frequency separation and found that this is not constant. From this parameter, we infer that the lower boundary condition of the electric field of the IAR is closest to a node, which agrees with previous studies. We compare the results of the non-uniform model with previous uniform models and evaluate their interpretations and the implications for the lower boundary condition. The ionosphere is the part of Earth's tenuous upper atmosphere which has been ionized by sunlight, resulting in the separation of heavier ions and free electrons into a state termed a "plasma." Like on a taut string, waves travel through the ionosphere along magnetic field lines reflecting from near the bottom of the ionosphere and back again at a boundary higher up. These waves can be observed in ground-based magnetic field data, and their frequencies can tell us about conditions in the ionosphere, including its plasma density distribution along the wave path. We have modeled these frequencies and then compared them with magnetic field measurements at an observatory in the UK. As the amount of plasma along the field line threading through the ionosphere changes, we can detect this as frequency variations and investigate how this affects the behavior of the waves. Solving a one-dimensional wave equation results in modeled harmonics of the Ionospheric Alfven ResonatorNon-uniform Alfven velocity profiles lead to different separations between the harmonicsFrom the offset of the fundamental frequency and average frequency separation, lower boundary conditions can be inferred

We extracted the harmonic frequency separation (Δ f ) of Ionospheric Alfvén Resonances (IAR) observed in the Eskdalemuir induction coil magnetometer data for the 9 year data set of 2013–2021. To obtain Δ f values, we used a machine learning technique that identifies the harmonics and from this we calculated the average separation. To investigate the climatology of the IAR, we have modeled the Δ f of the IAR for the data set using a time of flight calculation with model Alfvén velocity profiles. When analyzing Δ f from the model and data, we found that in general they follow the same trends. The modeled Δ f and Δ f from the data both show an inverse correlation with foF 2, which confirms that the frequencies of the IAR are controlled by electron density. It follows that Δ f is greater around midnight and during the winter months, due to the decrease in plasma mass density. Variability is also reflected when comparing yearly trends in Δ f with the sunspot number; higher frequencies are observed and modeled at low sunspot number. It is difficult to examine trends with instantaneous geomagnetic activity as IAR are not visible in spectrograms when geomagnetic activity is high. We find cases where the difference in measured and modeled Δ f is significant, suggesting that the model does not capture short term variations in plasma mass density that influence the IAR during these days. We plan to undertake further modeling of Δ f on shorter timescales.

Previous observational studies have shown that the natural Alfvén frequencies of geomagnetic field lines vary significantly over the course of a geomagnetic storm, decreasing by up to 50% from their quiet time values outside the plasmasphere. This was recently demonstrated statistically using ground magnetometer observations across 132 geomagnetic storm events (Wharton et al., 2020). This then brings into question where field line resonances (FLRs) will form in storm‐time conditions relative to quiet times. With storm‐time radiation belt dynamics depending heavily upon wave‐particle interactions, understanding how FLR locations change over the course of a storm will have important implications for this area. Using 3D magnetohydrodynamic (MHD) simulations, we investigate how changes in the Alfvén frequency continuum of the Earth's dayside magnetosphere over the course of a geomagnetic storm affect the fast‐Alfvén wave coupling. By setting the model Alfvén frequencies consistent with the observations, and permitting a modest change in the plasmapause/magnetopause locations consistent with storm‐time behavior, we show that FLR locations can change substantially during storms. The combined effects of higher fast waveguide frequencies and lower Alfvén frequencies during storm main phases, act together to move the FLR locations radially inwards compared to quiet times. FLRs outside of the plasmasphere are moved radially inward by 1.7 Earth radii for the cases considered.

Geomagnetic storms are one of the most energetic space weather phenomena. Previous studies have shown that the eigenfrequencies of ultralow frequency (ULF) waves on closed magnetic field lines in the inner magnetosphere decrease during storm times. This change suggests either a reduction in the magnetic field strength and/or an increase in its plasma mass density distribution. We investigate the changes in local eigenfrequencies by applying a superposed multiple-epoch analysis to cross-phase spectra from 132 geomagnetic storms. Six ground magnetometer pairs are used to investigate variations from approximately 3 L L > 4, the eigenfrequencies decrease by as much as 50% relative to their quiet time values. Both a decrease in magnetic field strength and an increase in plasma mass density, in some locations by more than a factor of 2, are responsible for this reduction. The enhancement of the ring current and an increase in oxygen ion density could explain these observations. AtL < 4, the eigenfrequencies increase due to the decrease in plasma mass density caused by plasmaspheric erosion.

Standing ultralow frequency waves redistribute energy and momentum around the Earth's magnetosphere. The eigenfrequencies of these standing waves can be measured by applying the cross-phase technique to ground magnetometer data. To make a detection, the flux tubes in the vicinity of the magnetometers must all be driven at their local eigenfrequencies by a source with a sufficient frequency width. Therefore, successful measurement of the local eigenfrequencies indicates that a broadband source is exciting the flux tubes. We have analyzed 10 years of magnetometer data with an automated cross-phase algorithm and used correlations with the OMNI data set to understand under what conditions broadband excitation occurs and how the conditions affect the eigenfrequency values. This is the largest such survey of its kind to date. We found that lower eigenfrequencies at higher latitudes (L > 5) and higher eigenfrequencies at lower latitudes (L < 4) were excited under different conditions. It was also possible to directly compare the first and third harmonics at midlatitudes. The lower eigenfrequencies were excited during more disturbed conditions, and we suggest that these harmonics are driven by solar wind pressure pulses or the Kelvin-Helmholtz instability at the magnetopause. The higher eigenfrequencies were excited when the magnetosphere was relatively quiet, and we suggest that the cause was waves generated upstream of the Earth's bow shock. The eigenfrequencies were observed to decrease in the middle magnetosphere during disturbed intervals. This is because the intensification of the ring current weakens the magnetic field. Variations in magnetic local time and latitude were also investigated.

The eigenfrequencies of standing Alfvén waves on closed magnetospheric field lines can be estimated using the cross‐phase technique. These eigenfrequencies can be used to monitor the plasma mass density distribution along the field line. So far, this has only been applied to ground‐based magnetometer data. The Super Dual Auroral Radar Network (SuperDARN) radars offer some benefits over magnetometers. They provide greater spatial resolution and coverage, as well as direct sensing above the E region ionosphere, which screens ultralow frequency (ULF) waves from the ground. However, there are significant data quality issues. These include the uncertain origin of radar backscatter, uneven sampling of data due to data gaps, and inaccurate fitting to the autocorrelation functions. Artificial backscatter from an ionospheric heater has been used to remove the uncertainty in backscatter location. We have developed a Lomb‐Scargle cross‐phase analysis for application to discontinuous radar data. The First Principles Fitting Methodology has been used to improve the fitted data products derived from the autocorrelation functions. Using these techniques, we have shown that it is possible to measure eigenfrequencies with SuperDARN data, and we have verified an example using ground‐based magnetometer data. Finally, we have demonstrated that the eigenfrequency signature in this example was caused by a broadband source of energy.

We have used EISCAT Svalbard Radar data, obtained during the International Polar Year 2007 campaign, to study ionospheric upflow events with fluxes exceeding 1013 m−2 s−1. In this study, we have classified the upflow events into low, medium, and high flux upflows, and we report on the incidence and seasonal distribution of these different classes. It is observed that high upflow fluxes are comparatively rare and low flux upflow events are a frequent phenomenon. Analysis shows that occurrence peaks around local noon at 31%, 16%, and 2% for low, medium, and high‐flux upflow, respectively, during geomagnetically disturbed periods. In agreement with previous studies on vertical and field‐aligned flows, ion upflow is observed to take place over a wide range of geomagnetic conditions, with downflow flux occurrence being lower than upflow occurrence. In contrast to previous observations, however, the upflow occurrence is greater around noon during highly disturbed geomagnetic conditions than for moderate geomagnetic conditions. Analysis of the seasonal distribution reveals that, while high‐flux upflow has its peak around local noon in the summer, with its occurrence being driven predominantly by high geomagnetic disturbance, the occurrence of low‐flux upflow is broadly distributed across all seasons, geomagnetic activity conditions, and times of day. The medium‐flux upflow events, although distributed across all seasons, show an occurrence peak strongly related to high Kp. Furthermore, during highly disturbed conditions, the low‐flux and medium‐flux upflow events show a minimum occurrence during the winter, whereas minimum occurrence for the high‐flux upflow events occurs in autumn.

Latitudinally spaced ground-based magnetometers can be used to estimate the eigenfrequencies of magnetic field lines using the cross-phase technique. These eigenfrequencies can be used with a magnetic field model and an assumed plasma mass density distribution to determine the plasma mass density in the magnetosphere. Automating this process can be difficult, and so far, it has not been possible to distinguish between the different harmonics. Misidentification of the harmonic mode will lead to incorrect estimations of the plasma mass density. We have developed an algorithm capable of identifying multiple harmonics in cross-phase spectrograms, using International Monitor for Auroral Geomagnetic Effects magnetometers. Knowledge of multiple harmonics allows the distribution of plasma mass density to be estimated instead of assumed. A statistical study was performed that showed clear common bands of eigenfrequencies, interpreted as different harmonics. These eigenfrequencies were lowest in the early afternoon and at higher latitudes. There was also a greater occurrence of measurements in the dayside. We then modeled the plasma mass density distribution with a power law characterized by the exponent p and compared the model eigenfrequencies to the data. This suggested that the even modes did not form during the interval of this study. Examination of the harmonic spacing and the high occurrence of the third harmonic supported this suggestion. We attribute the absence of the even modes to the driving mechanisms. Finally, we show that an equatorial bulge in plasma mass density was not present in our study.

This is the 700 m.a.s.l. contour lne generated from the DEM of the project area. Random permutations of DGB sites were generated above this elevational contraint.

Pc3 pulsations (frequency ∼20–100 mHz) which originate in the ion foreshock upstream of the Earth's bow shock due to the interaction between reflected ions and the solar wind are frequently observed in ground‐based pulsation magnetometer data. Previous studies have noted increased Pc3 wave power in the vicinity of the dayside cusp and inferred that the upstream waves gained entry via the cusp, although more recent studies have revealed a more complex picture. Here, we examine Pc3 wave power near local noon observed by search coil magnetometers at three closely‐spaced stations on Svalbard, during times when an extended interval of HF radar backscatter indicative of the cusp is detected by the Hankasalmi SuperDARN radar. The location of the equatorward edge of the HF radar cusp may then be directly compared with the Pc3 wave power measured at three latitudes as the cusp migrates across the stations on a statistical basis. These observations are more consistent with wave entry to the magnetosphere along closed field lines equatorward of the cusp via the ionospheric transistor mechanism of Engebretson et al. (1991a), or weakly coupled fast and Alfvén wave modes, which then map to the low‐latitude boundary layer or outer magnetosphere, rather than with wave entry into the magnetosphere via the cusp proper or exterior cusp.

[1] Presented here is a discussion of the results of a superposed epoch analysis of geomagnetic storms over the last solar cycle. Storms, identified by means of their characteristic SYM-H evolution, are separated by size into weak (−150 < SYM-H ≤ −80) nT, moderate (−300 < SYM-H ≤ −150) nT, and intense (SYM-H ≤ −300) nT categories. Where possible, the corresponding solar wind (SW) onset mechanisms were located by means of 1 min ACE OMNI data. Intense storms were observed to be driven solely by coronal mass ejections (CMEs); moderate storms were dominated by CME onset, while only weak storms were driven by both CMEs and corotating interaction regions (CIRs) at a ratio of ∼2:1, respectively. As might be expected, more intense storms resulted from the largest SW enhancements. Individual storm phase durations for different storm sizes were investigated, revealing that the duration of the main phase increases with storm size to a critical point, then decreases for more intense storms, contrary to the findings of a previous study by Yokoyama and Kamide (1997). Various SW-magnetosphere coupling functions were investigated for this data set in an attempt to estimate storm size from SW conditions.

We present the results of coordinated satellite and ground-based observations of the high-latitude ionospheric phenomena induced by high-power high-frequency (HF) radio waves. The ion outflow phenomenon accompanied by a strong increase in the electron temperature and thermal expansion of plasma was observed in the evening hours, when the high-latitude ionospheric F region was heated by high-power O-mode HF radio waves. The DMSP F15 satellite recorded an increase in the ion number density O+ at an altitide of about 850 km in that period. Ultralow-frequency (ULF) radiation at the modulation frequency 3 Hz of the high-power HF radio waves, which was generated in the ionosphere irradiated by high-power O-mode HF radio waves and accompanied by a strong increase in the electron temperature and the generation of artificial small-scale ionospheric irregularities, was recorded by the CHAMP satellite during the heating experiment in Tromsø in November 5, 2009. The results of the DEMETER satellite observations of extremely low frequency (ELF) radiation at the modulation frequency 1178 Hz of the high-power radio waves in the heating experiments were analyzed using the event of March 3, 2009 as an example.

James Hutchinson and colleagues describe a novel radar technique to study geomagnetic storms: superposed latitude–time–velocity plots. This is a summary of the poster winning a Rishbeth Prize at the NAM/UKSP/MIST meeting in April.

We present the results of complex experiments dealing with the impact of powerful HF radiowaves on the high-latitude ionosphere using the European Incoherent Scatter Scientific Association (EISCAT) facilities. During the ionospheric F-region heating by powerful extraordinary (X-mode) polarized HF radiowaves under the conditions of heating near the critical f H frequency f H ≈ f x F2 of the extraordinary wave of the F2-layer, we were first to detect the excitation of intense artificial small-scale ionospheric irregularities (ASIs), accompanied by electron temperature increases by approximately 50%. The results of coordinated satellite and ground-based observations of the powerful HF radiowave impact on the high-latitude ionosphere are considered. During ionospheric F-region heating by powerful HF radiowaves of ordinary polarization (O-mode) during evening hours, the phenomenon of ion outflow accompanied by electron temperature increases and thermal plasma expansion was revealed. Concurrent DMSP-F15 satellite measurements at a height of about 850 km indicate an O+ ion density increase. The CHAMP satellite observations identified ULF emissions at the modulation frequency (3 Hz) of the powerful HF radiowave, generated during modulated emissions of the powerful HF radiowave of O-polarization and accompanied by a substantial increase in the electron temperature and ASI generation.