Understanding the controlling factors of Ultra-Low Frequency waves and their penetration during geomagnetic storms

Wave-particle interactions play a key role in radiation belt dynamics. Traditionally, Ultra-Low Frequency (ULF) wave-particle interaction is parameterised statistically by a small number of controlling factors for given solar wind driving conditions or geomagnetic activity levels. Here, we investigate solar wind driving of ultra-low frequency (ULF) wave power and the role of the magnetosphere in screening that power from penetrating deep into the inner magnetosphere. We demonstrate that, during enhanced ring current intensity, the Alfv&#233;n continuum plummets, allowing lower frequency waves to penetrate deeper into the magnetosphere than during quiet periods. With this penetration, ULF wave power is able to accumulate closer to the Earth than characterised by statistical models. During periods of enhanced solar wind driving such as coronal mass ejection driven storms, where ring current intensities maximise, the observed penetration provides a simple physics-based reason for why storm-time ULF wave power is different compared to non-storm time waves. We demonstrate statistically that the ring current plays a pivotal role in allowing ULF wave energy to access the inner magnetosphere and show a new parameterisation of ULF wave power for radiation belt research purposes that is specifically tuned for geomagnetic storms.

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The magnetospheric interactions of predicted ULF wave power

10.5194/egusphere-egu21-15528 ◽

2021 ◽

Author(s):

Sarah Bentley ◽

Rhys Thompson ◽

Clare Watt ◽

Jennifer Stout ◽

Teo Bloch

Keyword(s):

Solar Wind ◽

Radiation Belt ◽

Internal State ◽

Low Frequency ◽

Ulf Waves ◽

Wave Power ◽

Ulf Wave ◽

Radiation Belt Electrons ◽

Spatial Parameters ◽

The Given

We present and analyse a freely-available model of the power found in ultra-low frequency waves (ULF, 1-15 mHz) throughout Earth&#8217;s magnetosphere. Predictions can be used to test our understanding of magnetospheric dynamics, while accurate models of these waves are required to characterise the energisation and transport of radiation belt electrons in space weather.This model is constructed using decision tree ensembles, which iteratively partition the given parameter space into variable size bins. Wave power is determined by physical driving parameters (e.g. solar wind properties) and spatial parameters of interest (magnetic local time MLT, magnetic latitude and frequency). As a parameterised model, there is no guarantee that individual physical processes can be extracted and analysed. However, by iteratively considering smaller scale driving processes, we identify predominant wave drivers and find that solar wind driving of ULF waves are moderated by internal magnetospheric conditions. Significant remaining uncertainty occurs with mild solar wind driving, suggesting that the internal state of the magnetosphere should be included in future.Models such as this may be used to create global magnetospheric &#8220;maps&#8221; of predicted wave power which may then be used to create radial diffusion coefficients determining the effect of ULF waves on radiation belt electrons.

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ULF Wave Power During Geomagnetic Storms and Implications for Radial Diffusion Processes

10.5194/egusphere-egu21-11203 ◽

2021 ◽

Author(s):

Jasmine Sandhu ◽

Jonathan Rae ◽

John Wygant ◽

Aaron Breneman ◽

Sheng Tian ◽

...

Keyword(s):

Statistical Analysis ◽

Diffusion Coefficients ◽

Radiation Belt ◽

Geomagnetic Storms ◽

Radial Diffusion ◽

Ulf Waves ◽

Wave Power ◽

Outer Radiation Belt ◽

Large Variability ◽

Ulf Wave

Ultra Low Frequency (ULF) waves drive radial diffusion of radiation belt electrons, where this process contributes to and, at times, dominates energisation, loss, and large scale transport of the outer radiation belt. In this study we quantify the changes and variability in ULF wave power during geomagnetic storms, through a statistical analysis of Van Allen Probes data for the time period spanning 2012 &#8211; 2019. The results show that global wave power enhancements occur during the main phase, and continue into the recovery phase of storms. Local time asymmetries show sources of ULF wave power are both external solar wind driving as well as internal sources from coupling with ring current ions and substorms.The statistical analysis demonstrates that storm time ULF waves are able to access lower L values compared to pre-storm conditions, with enhancements observed within L = 4. We assess how magnetospheric compressions and cold plasma distributions shape how ULF wave power propagates through the magnetosphere. Results show that the Earthward displacement of the magnetopause is a key factor in the low L enhancements. Furthermore, the presence of plasmaspheric plumes during geomagnetic storms plays a crucial role in trapping ULF wave power, and contributes significantly to large storm time enhancements in ULF wave power.The results have clear implications for enhanced radial diffusion of the outer radiation belt during geomagnetic storms. Estimates of storm time radial diffusion coefficients are derived from the ULF wave power observations, and compared to existing empirical models of radial diffusion coefficients. We show that current Kp-parameterised models, such as the Ozeke et al. [2014] model, do not fully capture the large variability in storm time radial diffusion coefficients or the extent of enhancements in the magnetic field diffusion coefficients.

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The influence of solar wind variability on magnetospheric ULF wave power

Annales Geophysicae ◽

10.5194/angeo-33-697-2015 ◽

2015 ◽

Vol 33 (6) ◽

pp. 697-701 ◽

Cited By ~ 3

Author(s):

D. Pokhotelov ◽

I. J. Rae ◽

K. R. Murphy ◽

I. R. Mann

Keyword(s):

Solar Wind ◽

Dynamic Properties ◽

Solar Wind Speed ◽

Low Frequency ◽

Wave Power ◽

Ion Temperature ◽

Ulf Wave ◽

Wind Variability ◽

Resonant Interactions ◽

Solar Wind Parameters

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.

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Solar cycle evolution of ULF wave power in solar wind and on ground

Journal of Space Weather and Space Climate ◽

10.1051/swsc/2020046 ◽

2020 ◽

Vol 10 ◽

pp. 43

Author(s):

Reko Hynönen ◽

Eija I. Tanskanen ◽

Patrizia Francia

Keyword(s):

Solar Wind ◽

Solar Cycle ◽

Solar Wind Speed ◽

Low Frequency ◽

Auroral Zone ◽

Wave Power ◽

Polar Cap ◽

Ulf Wave ◽

Substorm Activity ◽

Ground Magnetic

The solar cycle evolution of the ultra-low frequency (ULF) power was studied in solar wind and on ground. We aim finding out how the ULF power in interplanetary and on ground magnetic field evolves over the solar cycle 23 (SC23) and how well do they follow each other in monthly time scales. The hourly power of the ULF waves was computed in the Pc5 frequency range 2–7 mHz for years 1998–2008. The highest wave power in SC23 is found to occur in late 2003 and the lowest at the solar minimum. Ground ULF power follows the IMF power and solar wind speed, particularly well during declining phase. The ULF power in winter exceeds the ULF power in other seasons during the declining phase of SC23, while equinoxes dominate in the ascending phase and the solar maximum. The ground ULF power was found to rise with magnetic latitude from 54° to 73°, after which Pc5 power decreases towards the polar cap. The Pc5 power in the auroral zone is larger in the nightside than the dayside due to substorm activity implying that magnetotail processes are an important contributor to the nightside ULF power.

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On the effect of ULF waves on the outer radiation belt during geomagnetic storms

10.5194/egusphere-egu21-5977 ◽

2021 ◽

Author(s):

Christopher Lara ◽

Pablo S. Moya ◽

Victor Pinto ◽

Javier Silva ◽

Beatriz Zenteno

Keyword(s):

Relativistic Electron ◽

Radiation Belt ◽

Geomagnetic Storms ◽

Cumulative Effect ◽

Radiation Belts ◽

Ulf Waves ◽

Relativistic Electrons ◽

Relative Role ◽

Outer Radiation Belt ◽

Ulf Wave

The inner magnetosphere is a very important region to study, as with satellite-based communications increasing day after day, possible disruptions are especially relevant due to the possible consequences in our daily life. It is becoming very important to know how the radiation belts behave, especially during strong geomagnetic activity. The radiation belts response to geomagnetic storms and solar wind conditions is still not fully understood, as relativistic electron fluxes in the outer radiation belt can be depleted, enhanced or not affected following intense activity. Different studies show how these results vary in the face of different events. As one of the main mechanisms affecting the dynamics of the radiation belt are wave-particle interactions between relativistic electrons and ULF waves. In this work we perform a statistical study of the relationship between ULF wave power and relativistic electron fluxes in the outer radiation belt during several geomagnetic storms, by using magnetic field and particle fluxes data measured by the Van Allen Probes between 2012 and 2017. We evaluate the correlation between the changes in flux and the cumulative effect of ULF wave activity during the main and recovery phases of the storms for different position in the outer radiation belt and energy channels. Our results show that there is a good correlation between the presence of ULF waves and the changes in flux during the recovery phase of the storm and that correlations vary as a function of energy. Also, we can see in detail how the ULF power change for the electron flux at different L-shell We expect these results to be relevant for the understanding of the relative role of ULF waves in the enhancements and depletions of energetic electrons in the radiation belts for condition described.

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Foreshock wave transmission into the magnetosheath and magnetosphere: results from global hybrid-Vlasov simulations

10.5194/egusphere-egu21-7255 ◽

2021 ◽

Author(s):

Lucile Turc ◽

Markus Battarbee ◽

Urs Ganse ◽

Andreas Johlander ◽

Yann Pfau-Kempf ◽

...

Keyword(s):

Solar Wind ◽

Mach Number ◽

Cone Angle ◽

Low Frequency ◽

Compressional Wave ◽

Wave Transmission ◽

Wave Power ◽

Solar Wind Flow ◽

Magnetic Pulsations ◽

Wave Properties

The foreshock, extending upstream of the quasi-parallel shock and populated with shock-reflected particles, is home to intense wave activity in the ultra-low frequency range. The most commonly observed of these waves are the &#8220;30 s&#8221; waves, fast magnetosonic waves propagating sunward in the plasma rest frame, but carried earthward by the faster solar wind flow. These waves are thought to be the main source of Pc3 magnetic pulsations (10 &#8211; 45 s) in the dayside magnetosphere. A handful of case studies with suitable spacecraft conjunctions have allowed simultaneous investigations of the wave properties in different geophysical regions, but the global picture of the wave transmission from the foreshock through the magnetosheath into the magnetosphere is still not known. In this work, we use global simulations performed with the hybrid-Vlasov model Vlasiator to study the Pc3 wave properties in the foreshock, magnetosheath and magnetosphere for different solar wind conditions. We find that in all three regions the wave power peaks at higher frequencies when the interplanetary magnetic field strength is larger, consistent with previous studies. While the transverse wave power decreases with decreasing Alfv&#233;n Mach number in the foreshock, the compressional wave power shows little variation. In contrast, in the magnetosheath and the magnetosphere, the compressional wave power decreases with decreasing Mach number. Inside the magnetosphere, the distribution of wave power varies with the IMF cone angle. We discuss the implications of these results for the propagation of foreshock waves across the different geophysical regions, and in particular their transmission through the bow shock.

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Magnetospheric Response to Solar Wind Forcing: ULF Wave – Particle Interaction Perspective

10.5194/angeo-2021-57 ◽

2021 ◽

Author(s):

Qiugang Zong

Keyword(s):

Solar Wind ◽

Radiation Belt ◽

Ring Current ◽

Dynamic Pressure ◽

Plasma Heating ◽

Interplanetary Shock ◽

Wind Forcing ◽

Ulf Waves ◽

Radiation Belt Electrons ◽

The Impact

Abstract. Solar wind forcing, e.g. interplanetary shock and/or solar wind dynamic pressure pulses impact on the Earth’s magnetosphere manifests many fundamental important space physics phenomena including producing electromagnetic waves, plasma heating and energetic particle acceleration. This paper summarizes our present understanding of the magnetospheric response to solar wind forcing in the aspects of radiation belt electrons, ring current ions and plasmaspheric plasma physics based on in situ spacecraft measurements, ground-based magnetometer data, MHD and kinetic simulations. Magnetosphere response to solar wind forcing, is not just a “one-kick” scenario. It is found that after the impact of solar wind forcing on the Earth’s magnetosphere, plasma heating and energetic particle acceleration started nearly immediately and could last for a few hours. Even a small dynamic pressure change of interplanetary shock or solar wind pressure pulse can play a non-negligible role in magnetospheric physics. The impact leads to generate series kind of waves including poloidal mode ultra-low frequency (ULF) waves. The fast acceleration of energetic electrons in the radiation belt and energetic ions in the ring current region response to the impact usually contains two contributing steps: (1) the initial adiabatic acceleration due to the magnetospheric compression; (2) followed by the wave-particle resonant acceleration dominated by global or localized poloidal ULF waves excited at various L-shells. Generalized theory of drift and drift-bounce resonance with growth or decay localized ULF waves has been developed to explain in situ spacecraft observations. The wave related observational features like distorted energy spectrum, boomerang and fishbone pitch angle distributions of radiation belt electrons, ring current ions and plasmaspheric plasma can be explained in the frame work of this generalized theory. It is worthy to point out here that poloidal ULF waves are much more efficient to accelerate and modulate electrons (fundamental mode) in the radiation belt and charged ions (second harmonic) in the ring current region. The results presented in this paper can be widely used in solar wind interacting with other planets such as Mercury, Jupiter, Saturn, Uranus and Neptune, and other astrophysical objects with magnetic fields.

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