Ultra-low Frequency Wave Activity Research Articles

On the Source of the Anomalous ULF Waves Detected at Both Ground and Space‐Borne Data on 23 June 2020

AbstractWe have analyzed a highly monochromatic (f = 1.67 mHz) and large‐amplitude Ultra Low Frequency (ULF) wave event observed at satellites and ground observatories on 23 June 2020 during super solar quiet geomagnetic conditions. The train wave was detected between 6:22 and 7:55 UT across a wide longitudinal range of ground stations from low to high latitudes. Using Deep Space Climate Observatory and THEMIS‐B spacecraft, which were in the interplanetary medium, we have identified the possible driver of such global ULF wave activity in the impact of a small pressure pulse accompanied by a discontinuity in the magnetic field. The prolonged duration (90 min) of the ULF waves as well as their latitude‐independent frequency and the small azimuthal wave number (m ∼ 0−2) can be explained in terms of a global magnetospheric waveguide mode. The amplitude and cross‐phase analysis of the wave activity at ground, together with the polarization pattern, suggest the waveguide mode coupling with field line resonance. Nevertheless, during the same time interval, indirect evidence exists of a rapid reconfiguration of the magnetotail in the form of Pi2 waves in the night‐side region. The analysis of energetic particle flux at ionospheric height (500 km) shows a direct connection between the ULF wave activity and particle precipitation.

Switchback Boundary Dissipation and Relative Age

Abstract We examine Parker Solar Probe (PSP) magnetic field and plasma observations during its first encounter with the Sun in early 2018 November. During this perihelion time, impulsive reversals in the magnetic field, called “switchbacks,” were found in the data set characterized by a quick rotation in B along with a simultaneous increase in solar wind flow. In this work, we examine the structure and morphology of 920 switchback boundaries as PSP enters and exits the structures, specifically looking for evidence of boundary degradation, dissipation, and associated ultralow frequency (ULF) magnetic wave activity. We find that boundaries with the most abrupt, step-function-like change in B r and V r also show little evidence of dissipation and ULF wave activity. In contrast, there is a set of boundaries that appears highly degraded with ULF magnetic activity in the vicinity of the boundary. We thus infer that the steep, step-like boundaries with little ULF activity are relatively young in comparison to the degraded boundaries. The distribution in relative ages suggests that the switchback boundary formation process is dynamic and evolving, even occurring near the PSP observation point inside of 40 R s .

Ultralow‐Frequency Waves in Driving Jovian Aurorae Revealed by Observations From HST and Juno

AbstractLarge‐scale electrical currents and Alfvénic waves are the two main drivers responsible for producing planetary aurorae. The relative contribution of each process is a central question in terrestrial auroral science, and poorly understood for other planets due to the relatively rare opportunity of in‐situ spacecraft measurements. Here, we present observations of Jupiter's aurorae from the Hubble Space Telescope (HST) contemporaneous with Juno magnetometer measurements in the magnetosphere. For three successive days, we found that the magnetospheric ultralow‐frequency (ULF) wave activity (with periods of 1–60 min) was correlated with auroral power. This was especially true for the Alfvénic modes. We further performed a statistical analysis based on HST visits during Juno's third and seventh orbit, which revealed a systematic correlation between ULF wave and auroral activity. Our results imply that Alfvénic wave power could be an important source in driving Jupiter's aurorae, as theoretically predicted.

ULF Wave Activity Observed in the Nighttime Ionosphere Above and Some Hours Before Strong Earthquakes

AbstractWe examine the relationship between earthquakes and ultralow frequency (ULF) wave activity in the nighttime ionosphere based on the electric field data in the direct current/ULF range observed by the DEMETER satellite over a ~5.5 year period from May 2005 to November 2010. ULF wave activity is identified by an automatic detection algorithm and those which occur on the geomagnetic disturbed days (Kp > 3 at any time intervals) are discarded. Only the earthquakes with depth ≤70 km and occurring in the region of |MLat| < 40° are selected. A superposed epoch analysis is performed to study the statistical association between ULF wave activity and the selected earthquakes. The results show that (1) there are clearly temporal and spatial correlations between ULF wave activity and earthquakes whose catalog magnitudes are both ≥4.8 and ≥5.0, and (2) enhanced ULF wave occurrence rate happens ~1 day and 1 week before the earthquakes and at less than 200 km distance from the epicenters.

Features of Nightside ULF Wave Activity in the Ionosphere

AbstractWe investigate the features of nightside ultralow frequency (ULF) waves in the ionosphere using electric field data in the DC/ULF range observed by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) satellite over a ~5.5‐year period from May 2005 to November 2010. The ULF wave events are recognized by an automatic detection algorithm, and a superposed epoch analysis is performed to study the characteristics of ULF waves during geomagnetic storms. The results show that (1) ULF waves of both nonmagnetospheric and magnetospheric origins occur in the ionosphere; (2) nonmagnetospheric ULF waves present seasonal variations but have no clear response to storms; (3) magnetospheric ULF waves show clear association with the recovery phase of storms but have no obvious seasonal variations; and (4) an interhemispheric asymmetry with higher magnetospheric ULF wave occurrence rate in the Southern Hemisphere particularly around storms can be partly explained by the fact that more of selected isolated storms occurred during summer in the Northern Hemisphere (NH), hence higher conductivity in the NH. However, an analysis of few storms during equinoxes also shows a minor asymmetry. These results still indicate that the ionospheric conductivity in the Southern Hemisphere would be lower than in the NH at nighttime.

Contribution of ULF Wave Activity to the Global Recovery of the Outer Radiation Belt During the Passage of a High‐Speed Solar Wind Stream Observed in September 2014

AbstractEnergy coupling between the solar wind and the Earth's magnetosphere can affect the electron population in the outer radiation belt. However, the precise role of different internal and external mechanisms that leads to changes of the relativistic electron population is not entirely known. This paper describes how ultralow frequency (ULF) wave activity during the passage of Alfvénic solar wind streams contributes to the global recovery of the relativistic electron population in the outer radiation belt. To investigate the contribution of the ULF waves, we searched the Van Allen Probes data for a period in which we can clearly distinguish the enhancement of electron fluxes from the background. We found that the global recovery that started on 22 September 2014, which coincides with the corotating interaction region preceding a high‐speed stream and the occurrence of persistent substorm activity, provides an excellent scenario to explore the contribution of ULF waves. To support our analyses, we employed ground‐ and space‐based observational data and global magnetohydrodynamic simulations and calculated the ULF wave radial diffusion coefficients employing an empirical model. Observations show a gradual increase of electron fluxes in the outer radiation belt and a concomitant enhancement of ULF activity that spreads from higher to lower L‐shells. Magnetohydrodynamic simulation results agree with observed ULF wave activity in the magnetotail, which leads to both fast and Alfvén modes in the magnetospheric nightside sector. The observations agree with the empirical model and are confirmed by phase space density calculations for this global recovery period.

Global Distribution of ULF Waves During Magnetic Storms: Comparison of Arase, Ground Observations, and BATSRUS + CRCM Simulation

AbstractsDuring 26–29 March 2017 magnetic storm, the Arase satellite observed typical ultra low frequency (ULF) waves and acceleration of relativistic electrons. We simulate the global distribution of these ULF waves using CRCM with BATSRUS global magnetospheric magnetohydrodynamic model. The simulation can qualitatively reproduce the ULF waves observed by Arase at frequencies of 2–3 mHz. However, the simulated ULF wave power is 1–2 orders of magnitude smaller than the observation. The simulated ULF wave activity has a good correlation with the solar‐wind dynamic pressure variation, while the wave activity on the ground is enhanced even during the recovery phase, possibly due to the Kelvin‐Helmholtz instability and/or substorms. We also study the 3–6 April 2017 magnetic storm, in which low ULF wave activity and weak acceleration of relativistic electrons are seen. We suggest that the existence of ULF waves plays an important role in accelerating electrons up to relativistic energies.

ULF Wave‐Associated Density Irregularities and Scintillation at the Equator

AbstractThis paper presents independent multi‐instrument observations that address the physical mechanisms of how ultralow‐frequency (ULF) wave‐associated electric fields initiate ionospheric density fluctuation and scintillation at the equator. Since the magnetic field at the equator is entirely embedded in a relatively high‐collision and high‐conductivity medium, the condition may not be possible for the geomagnetic field to fluctuate due to ULF wave activity. This implies that the fluctuating electric field at the equator may not be produced through equatorial dynamo action due to fluctuating magnetic fields. Instead, the electric field penetrates from high latitudes and produces fluctuating magnetic field as well as modulates the vertical drift and hence causes the density to fluctuate at the equatorial region. We demonstrate this by estimating the ULF associated fluctuating electric field at high latitudes and at the equatorial region by applying the appropriate attenuation factor as it penetrates to lower latitudes. The periodicity of both electric field and density fluctuations appears to be between 6 and 9 min, which is a typical period of ULF waves in the Pc5 range. Because of its large amplitude and long periods compared to other ULF wave frequency bands, the Pc5 wave‐associated electric field, which can even be estimated using magnetograms with low sensitivity and low sampling rate (e.g., 1 min), can easily penetrate to the lower latitude region and produce significant ionospheric density fluctuations that can be strong enough to create scintillation at the equatorial region.

Investigation of ∼ 20–40 mHz ULF waves and their driving mechanisms in Mercury's dayside magnetosphere

Abstract. Ultra-low-frequency (ULF) waves in the ∼ 20–40 mHz range are frequently observed in the Mercury magnetosphere using Mercury Surface Space Environment Geochemistry, and Ranging (MESSENGER) magnetic field data. The majority of these waves have very similar characteristics to the waves likely driven by Kelvin–Helmholtz (KH) ULF waves (which are retained as a subset of the wave events studied in this paper) identified in a previous study. Significant ULF wave activity is observed in the dawn sector of the magnetosphere. This indicates that Mercury KH waves may be more common between 6 and 12 magnetic local time than previously predicted and that magnetospheric ULF waves in the frequency band ∼ 20–40 mHz can be used as a detection tool for Hermean KH waves.

Association of radiation belt electron enhancements with earthward penetration of Pc5 ULF waves: a case study of intense 2001 magnetic storms

Abstract. Geospace magnetic storms, driven by the solar wind, are associated with increases or decreases in the fluxes of relativistic electrons in the outer radiation belt. We examine the response of relativistic electrons to four intense magnetic storms, during which the minimum of the Dst index ranged from −105 to −387 nT, and compare these with concurrent observations of ultra-low-frequency (ULF) waves from the trans-Scandinavian IMAGE magnetometer network and stations from multiple magnetometer arrays available through the worldwide SuperMAG collaboration. The latitudinal and global distribution of Pc5 wave power is examined to determine how deep into the magnetosphere these waves penetrate. We then investigate the role of Pc5 wave activity deep in the magnetosphere in enhancements of radiation belt electrons population observed in the recovery phase of the magnetic storms. We show that, during magnetic storms characterized by increased post-storm electron fluxes as compared to their pre-storm values, the earthward shift of peak and inner boundary of the outer electron radiation belt follows the Pc5 wave activity, reaching L shells as low as 3–4. In contrast, the one magnetic storm characterized by irreversible loss of electrons was related to limited Pc5 wave activity that was not intensified at low L shells. These observations demonstrate that enhanced Pc5 ULF wave activity penetrating deep into the magnetosphere during the main and recovery phase of magnetic storms can, for the cases examined, distinguish storms that resulted in increases in relativistic electron fluxes in the outer radiation belts from those that did not.

The influence of solar wind variability on magnetospheric ULF wave power

Abstract. Magnetospheric ultra-low frequency (ULF) oscillations in the Pc 4–5 frequency range play an important role in the dynamics of Earth's radiation belts, both by enhancing the radial diffusion through incoherent interactions and through the coherent drift-resonant interactions with trapped radiation belt electrons. The statistical distributions of magnetospheric ULF wave power are known to be strongly dependent on solar wind parameters such as solar wind speed and interplanetary magnetic field (IMF) orientation. Statistical characterisation of ULF wave power in the magnetosphere traditionally relies on average solar wind–IMF conditions over a specific time period. In this brief report, we perform an alternative characterisation of the solar wind influence on magnetospheric ULF wave activity through the characterisation of the solar wind driver by its variability using the standard deviation of solar wind parameters rather than a simple time average. We present a statistical study of nearly one solar cycle (1996–2004) of geosynchronous observations of magnetic ULF wave power and find that there is significant variation in ULF wave powers as a function of the dynamic properties of the solar wind. In particular, we find that the variability in IMF vector, rather than variabilities in other parameters (solar wind density, bulk velocity and ion temperature), plays the strongest role in controlling geosynchronous ULF power. We conclude that, although time-averaged bulk properties of the solar wind are a key factor in driving ULF powers in the magnetosphere, the solar wind variability can be an important contributor as well. This highlights the potential importance of including solar wind variability especially in studies of ULF wave dynamics in order to assess the efficiency of solar wind–magnetosphere coupling.

ULF wave activity in high‐speed streams of the solar wind: Impact on the magnetosphere

AbstractIn recent years, the study of solar‐magnetospheric interaction has increasingly been focusing on wave phenomena and the role of turbulence in the processes of energy and momentum transfer from the Sun to the Earth's magnetosphere. Here we consider one aspect of the problem — the impact of wave patterns associated with high‐speed solar wind streams (HSSs) on the variable geomagnetic field and the trapped radiation. The superposed epoch method was chosen as our research technique. We use observations of the solar wind plasma and the interplanetary magnetic field at libration point L1, as well as measurements of electron fluxes at geostationary orbit and of geomagnetic pulsations on ground. Data from 31 corotating interaction regions (CIR), their sources being coronal holes on the Sun, and 13 streams from coronal mass ejections (CME) were processed. The focus is on the regime of ultra‐low frequency (ULF) waves in high‐speed flows and ULF fluctuations in the magnetosphere. A significant difference has been revealed between the wave activity characteristics of the two HSS types in the solar wind and on the earth. Irregular pulses are typical for coronal mass ejections and wave‐like conditions are common for CIRs. For the latter, the temporal profile of the normalized ULF amplitude in the solar wind is similar to the pulsation amplitude profile on the ground. We also found that a high level of ULF activity in the magnetosphere during CIR intervals is favorable to 2 MeV electron flux enhancement, but does not affect the flux of less energetic electrons.

Changes in the ultra-low frequency wave field during the precursor phase to the Sichuan earthquake: DEMETER observations

Abstract. Electromagnetic phenomena observed in association with increases in seismic activity have been studied for several decades. These phenomena are generated during the precursory phases of an earthquake as well as during the main event. Their occurrence during the precursory phases may be used in short-term prediction of a large earthquake. In this paper, we examine ultra-low frequency (ULF) electric field data from the DEMETER satellite during the period leading up to the Sichuan earthquake. It is shown that there is an increase in ULF wave activity observed as DEMETER passes in the vicinity of the earthquake epicentre. This increase is most obvious at lower frequencies. Examination of the ULF spectra shows the possible occurrence of geomagnetic pearl pulsations, resulting from the passage of atmospheric gravity waves generated in the vicinity of the earthquake epicentre.

Observations of ULF waves in the solar corona and in the solar wind at the Earth's orbit

Signs were looked for that would indicate a possible connection between plasma velocity oscillations observed in the region of solar coronal holes and magnetic field oscillations as recorded in the interplanetary medium. The problem appears to be quite important since the presence of large-scale ULF waves in the solar wind can increase geoeffectiveness of high speed streams in the interplanetary plasma. Observations of solar oscillations in the FeI 6569Å spectral line in a coronal hole were taken as a basis. The measurements were carried out at the Horizontal Automated Solar Telescope of the Sayan Solar Observatory. High speed solar wind stream ejected from the coronal hole reached the Earth's orbit after approximately 60h. The spectra of solar oscillations were compared with those of ultra low frequency (ULF) oscillations of the interplanetary magnetic field (IMF) at libration point L1. The oscillations were recorded with the ACE magnetometer when the leading edge of the high speed stream, bringing increased ULF wave activity, reached the Earth. The spectra of solar oscillations had a sharp peak at about 3.4–3.6mHz. The spectrum of the solar wind ULF oscillations is much more complex, being formed by different sources. Nevertheless, ULF oscillations of the IMF often had peaks that were close in frequency to those of the solar oscillations. Analysis of the ULF wave spectra observed in the 92 high speed streams confirmed the presence of 3- and 5-min oscillations in the total wave spectrum. It is emphasized that the results cannot be regarded as proving a direct connection between solar oscillations and ULF waves at the Earth's orbit even though they do support such a possibility. Additional research is needed involving IMF wave trajectory calculations.

THEMIS observations of ULF wave excitation in the nightside plasma sheet during sudden impulse events

Sudden impulses (SIs) are an important source of ultra low frequency (ULF) wave activity throughout the Earth's magnetosphere. Most SI‐induced ULF wave events have been reported in the dayside magnetosphere; it is not clear when and how SIs drive ULF wave activity in the nightside plasma sheet. We examined the ULF response of the nightside plasma sheet to SIs using an ensemble of 13 SI events observed by THEMIS (Timed History of Events and Macroscale Interactions during Substorms) satellites (probes). Only three of these events resulted in ULF wave activity. The periods of the waves are found to be 3.3, 6.0, and 7.6 min. East‐west magnetic and radial electric field perturbations, which typically indicate the toroidal mode, are found to be stronger and can have phase relationships consistent with standing waves. Our results suggest that the two largest‐amplitude ULF responses to SIs in the nightside plasma sheet are tailward‐moving vortices, which have previously been reported, and the dynamic response of cross‐tail currents in the magnetotail to maintain force balance with the solar wind, which has not previously been reported as a ULF wave driver. Both mechanisms could potentially drive standing Alfvén waves (toroidal modes) observed via the field‐line resonance mechanism. Furthermore, both involve frequency selection and a preference for certain driving conditions that can explain the small number of ULF wave events associated with SIs in the nightside plasma sheet.

ULF wave activity during the 2003 Halloween superstorm: multipoint observations from CHAMP, Cluster and Geotail missions

Abstract. We examine data from a topside ionosphere and two magnetospheric missions (CHAMP, Cluster and Geotail) for signatures of ultra low frequency (ULF) waves during the exceptional 2003 Halloween geospace magnetic storm, when Dst reached ~−380 nT. We use a suite of wavelet-based algorithms, which are a subset of a tool that is being developed for the analysis of multi-instrument multi-satellite and ground-based observations to identify ULF waves and investigate their properties. Starting from the region of topside ionosphere, we first present three clear and strong signatures of Pc3 ULF wave activity (frequency 15–100 mHz) in CHAMP tracks. We then expand these three time intervals for purposes of comparison between CHAMP, Cluster and Geotail Pc3 observations but also to be able to search for Pc4–5 wave signatures (frequency 1–10 mHz) into Cluster and Geotail measurements in order to have a more complete picture of the ULF wave occurrence during the storm. Due to the fast motion through field lines in a low Earth orbit (LEO) we are able to reliably detect Pc3 (but not Pc4–5) waves from CHAMP. This is the first time, to our knowledge, that ULF wave observations from a topside ionosphere mission are compared to ULF wave observations from magnetospheric missions. Our study provides evidence for the occurrence of a number of prominent ULF wave events in the Pc3 and Pc4–5 bands during the storm and offers a platform to study the wave evolution from high altitudes to LEO. The ULF wave analysis methods presented here can be applied to observations from the upcoming Swarm multi-satellite mission of ESA, which is anticipated to enable joint studies with the Cluster mission.

The correlation of ULF waves and auroral intensity before, during and after substorm expansion phase onset

We present case studies of the evolution of Ultra‐Low Frequency (ULF) magnetic wave amplitudes and auroral intensity through the late growth phase and the expansion phase of the substorm cycle. We present strong evidence that substorm‐related auroral enhancements are clearly and demonstrably linked to ULF wave amplitudes observed at the same location. In all three case studies presented, the correlation analysis shows that the ULF wave activity and auroral intensities are highly correlated at close to zero lag. We discuss four possible explanations that may be able to explain both the timing and the high correlations between these two phenomena, including a simple coincidence, an artifact of instrumental effects, the response of the ionosphere to magnetic waves and auroral particle precipitation, and finally, that ULF waves and auroral particle precipitation are physically linked at their source. We discount coincidence and instrumental effects since we present multiple events where instrumental effects have a negligible contribution, and we find that the ionospheric response to waves and precipitation can explain some, but not all, of the results contained within this paper. Specifically, the ionospheric response to substorm waves and auroral precipitation cannot explain the result backed up by previous studies that the onset of ULF wave activity and the onset of auroral particle precipitation occur at the same time and in the same location. This leaves the possibility that ULF waves and auroral particles are physically linked at their source. We therefore re‐emphasize the importance of ULF wave observations in fully understanding the mechanism or mechanisms responsible for rapid auroral brightenings.