Solar Wind Kinetic Energy Research Articles

Energetics of the magnetosphere during the magnetic storm

The most detailed studies of the energetic budget of the magnetosphere during the magnetic storms were done on the basis of the paraboloid model using the November 23–27, 1986 and May 6–8, 1988 magnetic storms. Calculations have shown that the energy injected in the course of the magnetic storms into the inner magnetosphere and ionosphere of both hemispheres amounts to ∼ 0.9 –2.2% of the solar wind kinetic energy on the magnetospheric cross section. The total energy injected into the magnetosphere from a distance of 60 R E in the tail down to the ionosphere is ∼4.0–7.5% of the solar wind kinetic energy during the main phase of the two magnetic storms. The injected energy into the tail E TL is 1.03–1.18 of the total energy input into the inner magnetosphere and ionosphere of both hemispheres at the main phase of the two storms. The decay parameter for the energy stored in the magnetospheric tail is ∼5 h . The total energy dissipated in the ionosphere of both hemispheres, in the inner magnetosphere and in the tail during the two storms, is 1.85×10 17 and 3.24×10 17 J , respectively. The total energy input into the magnetosphere is calculated to be 1.77×10 17 and 3.16×10 17 J . The discrepancies of 0.08×10 17 and 0.10×10 17 J amount to 4.3% and 3.1% of the total energy input and characterize the accuracy of the magnetospheric energy budget calculation. In the magnetotail the balance between the injected and dissipated energy of ∼1.09×10 17 J for one storm and ∼1.7×10 17 J for the other is preserved as well. We conclude that one-half of the energy which is injected into the magnetosphere from the solar wind during the storms enters the magnetotail and dissipates there. The coupling parameter ε PA is widely considered to be a measure of the energy dissipation in the inner magnetosphere. The dissipation energy in the inner magnetosphere U T= U J+ U A+ U DR is defined as the sum of the contributions of the Joule dissipation U J, the energy of auroral particle precipitation U A, and the energy injection into the ring current U DR . In this paper, we find that ε PA in the two storms investigated is substantially different from U T. The energy injected into the ring current region at the main phase of the storm amounts to ∼10 16 J . It is nearly 3 or 4 times smaller than the energy input into the magnetosphere via the field-aligned currents or the energy dissipated in the ionosphere by Joule dissipation. The energy injected into the magnetosphere is transferred mainly into processes different from the ring current generation. During the development of intensive auroral electrojets, the energy dissipation in the magnetotail and the increase in the energy of the tail current system occur simultaneously. The energy dissipation in the inner magnetosphere and ionosphere U S occurs not only at the expense of energy previously stored in the magnetotail, but rather at the expense of energy that is injected into the near-Earth tail. This energy transfer from the solar wind into the magnetotail and the energy dissipation in the ionosphere increased simultaneously. Thus, during disturbances in the magnetosphere, simultaneously loading–unloading and directly driven processes occur. Loading- and unloading processes manifest themselves both in the storage of the solar wind energy in the magnetotail and the ring current, and subsequent dissipation. The directly driven processes become manifest in the direct dissipation of the energy which enters into the ionosphere through large-scale field-aligned current systems.

Magnetospheric energy budget and the epsilon parameter

Determination of the energy input for the magnetospheric energy budget is a nontrivial matter. As no direct means to measure the input are known, various solar wind‐derived proxies have been developed. In this article we discuss one of the most widely used energy input functions, the so‐called epsilon parameter of Akasofu. While practice has shown it to be a very useful parameter, there is no convincing evidence that it is superior to all other coupling parameters. Furthermore, its somewhat unclear definition and lack of physical foundation sometimes lead to confusing interpretation of the parameter in practical studies of magnetospheric energy cycle. For example, the parameter is sometimes understood to describe the transfer of solar wind Poynting flux into the magnetosphere, whereas the actual physical energy transfer involves conversion of solar wind kinetic energy to magnetic energy measured inside the magnetopause. Another questionable interpretation is to relate the size of the energy transfer region to the length of the reconnection line, as the scale factor in epsilon has the physical unit of area. These confusions may partly result from mixing the concepts of energy source and energy transfer. In spite of these problems the present empirical formulation of the epsilon parameter appears, from the global energy budget point of view, to give a remarkably good estimate for the total energy input into the inner magnetosphere in substorm and storm timescales. This is even more remarkable as after the parameter was first formulated we have learned that the ionosphere is a major sink of storm and substorm energy, exceeding the ring current in importance as an energy output channel. An additional issue is the energy carried away by the plasmoids and outflow of the postplasmoid plasma sheet. One can argue that the application of epsilon should be restricted to the energy consumption in the inner magnetosphere. However, as the intermittent plasmoid releases are essential parts of the same complex of processes as the ring current enhancement and ionospheric particle injections, we argue that they should be included in the energy budget, even if that might result in rejection of the epsilon as a useful input parameter. The recent analyses of energy output suggest that we can still use epsilon by scaling the parameter up by a factor of 1.5–2. It should be noted, however, that this energy budget does not account for all energy passing through the magnetosphere but only that part which is consumed in the storm and substorm processes.

Energy analysis of substorms based on remote sensing techniques, solar wind measurements, and geomagnetic indices

Observations from the Polar Ionospheric X‐ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to examine the energy deposition in the Northern Hemisphere by precipitating electrons for seven substorms during 1997. By combining the results from these two remote sensing techniques we derive the 5 min average electron energy distributions from 100 eV to 100 keV with a spatial resolution of ∼700 km. During growth phase we find that most of the energy is carried by electrons below 10 keV. The study shows that the maximum intensity of the substorm is mostly defined by the flux of electrons below 10 keV. In contrast, the electrons above 10 keV show a greater intensification at substorm onset and are more confined azimuthally during the expansion phase. By using the most appropriate parameterized methods to estimate the energy increase of the ring current (UR) and the Joule heating rate in both hemispheres (UJ) and by adding the rate of energy deposition in both hemispheres by precipitating electrons (UA), we estimate the total energy dissipation rate during substorms (UT). Our estimate of UA is a factor 2–4 larger than that reported in earlier studies. We find that the contributions to the total time‐integrated energy dissipation over the duration of the substorm, W(UT), from W(UR), W(UJ), and W(UA) on average are 15%, 56%, and 29%. Comparing W(UT) and the time‐integrated ϵ parameter, W(ϵ), which approximates the solar wind input due to dayside reconnection, we find that the ϵ parameter does not always provide enough energy into the magnetosphere to balance W(UT). An additional energy transfer mechanism is needed to balance the energy budget. We find that a viscous interaction that transfers 0.17% of the solar wind kinetic energy in addition to the energy transfer due to dayside reconnection, estimated by the ϵ parameter, is sufficient to balance the total energy dissipation UT.

Accelerated particles from turbulent boundary layer

We study the high latitude turbulent boundary layer (TBL), where the essential magnetosheath (MSH) ion heating (in 1.5–3 times) is seen in more than 80 % cases within ‘diamagnetic bubbles’ -structures with highly reduced magnetic field. The high- energy tails of the core particle distributions in TBL exceed 20keV for ions and 10keV for electrons on April 21, 1996. We find indications also for further acceleration of a small fraction of protons there up to 300 keV. More energetic particles could have maximum intensity just inside magnetopause that is consistent with their magnetospheric origin. Simultaneous Polar and Interball-1 data in TBL and MSH on May 29, 1996 demonstrate heating of the MSH core ions in cusp, TBL and stagnant MSH. In the TBL the O+ ions might be heated as well (up to few keV) and even be backscattered towards the ionosphere together with the protons > 18 keV. The dominant incompressible Alfvenic waves in TBL constitute an intermediate chain for the transformation of the solar wind kinetic energy into the auroral particle acceleration/heating. The most intensive energy transformation occurs over cusps and along the boundary between the mantle and low latitude boundary layer down to X= − 6 RE. It provides primary heating for MSH ions in both cusp proper and boundary cusp.

Statistical studies of plasma waves and backstreaming electrons in the terrestrial electron foreshock observed by Geotail

We present statistical studies of the direction finding of 2ƒp radiation and the spatial distribution of plasma waves and energetic particles in the terrestrial electron foreshock observed by Geotail. First, we investigate the geometry of the electron foreshock which is assumed to be the “2ƒp radio source.” The 2ƒp radio source is likely to be in the leading region of the electron foreshock where the most intense Langmuir waves are observed. The Langmuir wave activities and the population of energetic electrons gradually decrease in the region beyond 10 RE from the contact point. The decreasing rate of Langmuir wave activity is very small, about 10−0.008/RE. We also find that in the region around the contact point of the tangential interplanetary magnetic field (IMF) lines and the bow shock, the observed amplitude of the 2ƒp radiation seems to become weak. We think that it is due to the weak activities of Langmuir waves in the region close to the contact point and/or the directivity of 2ƒp radiation along the tangential IMF line. Next, we investigate the influence of the solar wind conditions on the activities in the electron foreshock. We confirm a positive correlation of the 2ƒp radio activity with the solar wind kinetic energy flux and a decrease of 2ƒp radio activity with decreasing IMF cone angle resulting in IMF lines tangent to the far flank of the bow shock. The 2ƒp radio activity is more affected by both parameters than the amplitude of Langmuir waves is affected. This suggests that the 2ƒp radio emissivity is very sensitive to the energy of Langmuir waves as expected from the generation process of 2ƒp radiation. Such high sensitivity also supports the concentration of the radio emissivity in the leading region of the electron foreshock and the limitation of the radio source extension along the magnetic field line. We also reinvestigate the comparison between the terrestrial and Venusian foreshocks. The differences between them suggest the nonsimilarity of shocks with different sizes.

Long-Term Energy and Momentum Flux Fluctuations of the Solar Wind at 1AU

The paper is concerned with the variations of solar wind kinetic energy, ion thermal energy, ion flux and momentum as observed by near-Earth spacecraft over time scales from seconds to several days. The mean values of these fluxes and characteristics of their variability are discussed. The average variations of kinetic energy and momentum flux are about 10% of the mean values over a one-hour interval. Consideration is given to the differences in the energy and momentum flux for low-velocity and high-velocity solar wind streams. The discussion is extended with an analysis of very intense disturbances of energy and momentum due to the arrival of an interplanetary shock wave at Earth.

Influence of the solar wind/interplanetary medium on saturnian kilometric radiation.

Previous studies on the periodicities of the Saturnian kilometric radiation (SKR) suggested a considerable solar wind influence on the occurrence of SKR, so it was obvious to investigate the relationship between parameters of the solar wind/interplanetary medium and this Saturnian radio component. Voyager 2 data from the Plasma Science experiment, the Magnetometer experiment and the Planetary Radio Astronomy experiment were used to analyze the external control of SKR. Out of the examined quantities known to be important in controlling magnetospheric processes this investigation yielded a dominance of the solar wind momentum, ram pressure and kinetic energy flux, in stimulating SKR and controlling its activity and emitted energy, and confirmed the results of the Voyager 1 analysis.

Dayside long‐period magnetospheric pulsations: Solar wind dependence

The spectral power and occurrence rate of long‐period magnetospheric pulsations (predominantly fundamental mode toroidal Pc 5) observed by the electron beam experiment on board GEOS 2 are compared with IMF and solar wind parameters. No clear influences of IMF orientation and magnitude on pulsation amplitudes and occurrence rate are found. Significant correlations, however, do exist between the spectral power of pulsations and the solar wind bulk velocity, and between the spectral power and the solar wind kinetic energy flux. The sign of the latter correlation depends on the Kp index. For Kp = 0 the pulsation power decreases with increasing solar wind kinetic energy flux, whereas it increases for Kp ≥ 1. Our results are consistent with the Kelvin‐Helmholtz instability at the inner side of the low‐latitude boundary layer being the dominant mechanism for the generation of fundamental mode toroidal Pc 5 magnetospheric pulsations. Flux transfer events play only an inferior role as energy sources for these pulsations. The coupling efficiency of surface waves at the boundary layer to shear Alfvén waves near geostationary orbit seems to change significantly if the geostationary orbit is inside the plasmasphere.

Comment on “Explosive magnetic reconnection: Puzzle to be solved as the energy supply process for magnetospheric substorms?” by Syun‐Ichi Akasofu] The other side of the substorm

There is in magnetospheric physics today a controversy of sorts, involving the nature of the process by which the kinetic energy of the solar wind is tapped to power magnetospheric phenomena and related geomagnetic disturbances. The controversy is conveniently described in the language of signal processing [Akasofu, 1985]: an input signal composed of a suitable combination of solar wind variables is processed by the magnetosphere to produce an output signal consisting of a suitable combination of geomagnetic activity indices and/or magnetospheric variables. Akasofu advocates the view that the output signal, when properly measured, is a linear function of the input signal, and that the magnetosphere is therefore “directly driven” (i.e., it responds passively, after a fixed time lag, to a given solar wind energy input). In the alternative view the magnetosphere plays a more active role: the system response depends not only on incident solar wind variables (allowing a suitable time lag) but also on the recent dynamical history of the system. This second view is described by Akasofu as the “unloading” scenario because it corresponds to a physical model in which solar wind kinetic energy is collected and stored temporarily in the magnetic field of the magnetospheric tail and is subsequently— in a manner determined at least in part by internal magnetospheric processes—“ unloaded” to the ionosphere and inner magnetosphere to produce readily observable geomagnetic effects.

Interaction between an interplanetary shock and the Earth's magnetosphere on August 27, 1978: ISEE 1 electric field and ISEE 2 plasma observations

An interplanetary shock caused a rapid compression of the magnetosphere on August 27, 1978. The ISEE 1/ISEE 2 pair of satellites were in the frontside magnetosphere before the shock, and subsequently a magnetosonic wave front, the magnetopause, and the bow shock passed the ISEE satellites in a very short time. Previous papers determined boundary velocities from magnetic time delay measurements. These papers form a starting point for the study of electric field data during these events. By a combination of electric and magnetic field data it is possible to determine both the magnetosonic velocity and the magnetopause velocity, in good agreement with the magnetic time delay measurements. Furthermore, it is possible to establish with good confidence that during this compressional motion the E × B velocity was toward the magnetopause from both sides, ∼75 km s−1 from the earth side and ∼50 km s−1 from the sun side. A power conversion flux close to +400 W km−2 could be determined for the magnetopause. A sunward potential drop of 135–280 V was detected near the bow shock, and in this case the power conversion flux was negative, and in the range −(80–160 W km−2), or ∼10% of the solar wind kinetic energy flux.

ULF geomagnetic power near L = 4: 7. A conjugate area study of power controlled by solar wind quantities

Geomagnetic energies are computed at hourly intervals for three period bands (in the Pc 3–5 range) from horizontal component magnetometer data recorded at Pittsburg, New Hampshire (L ∼ 3.5), Girardville, Quebec (L ∼ 4.4), and their approximate conjugate location at Siple, Antarctica (L ∼ 4.0). The data were recorded during September 23–29, 1977, a relatively quiet to moderately disturbed geomagnetic interval. High correlations are found at all three stations between day side ground magnetic energies and the solar wind kinetic energy flux density by using multivariate analysis techniques. The interplanetary quantity ε, whose values did not exceed 4×1018 erg/s for this data set, is found not to have a significant association with day side magnetic activity. The similar results found at the conjugate stations indicate that the solar wind kinetic pressure variations are important to control dayside broadband magnetic fluctuations in the Pc 3–5 frequency range measured on the ground.

ULF geomagnetic power near L=4, 6. Relationship to upstream solar wind quantities

Geomagnetic energy densities, computed at hourly intervals in three frequency bands from magnetometer data at Pittsburg, New Hampshire, (L ∼3.5) for 28 days during July and August 1975, are correlated with upstream solar wind parameters obtained from instruments on the Imp J spacecraft. The diurnal ground magnetic energy variations generally show higher power levels for the lower frequency fluctuations. Enhancements in the power levels may appear at different local times on individual days. Dayside and nightside ground hourly magnetic energies are generally found to correlate best with the solar wind kinetic energy flux density fk=1/2npmpV³ from amongst the various upstream interplanetary quantities examined. The highest linear correlation coefficients found between power versus fk and power versus VSW are generally obtained during local afternoon hours, with a sharp reduction in correlation occurring at local noon. We conclude that different interplanetary source mechanisms contribute to the dayside ground power in different frequency bands. Improved correlations are expected for nightside power levels with geomagnetic tail quantities rather than with upstream solar wind quantities.

Models of coronal hole flows

Models of plasma flow in a coronal hole fall naturally into four classes. These are: (i) radial flow on a streamline along which the divergence is assumed to vary differently than as the square of the radial distance from the Sun; (ii) global flow along streamlines determined in some independent manner; (iii) empirical models originating in, or based strongly on observation; (iv) dynamic models using magnetic and plasma boundary conditions low in the corona to find both the geometry of streamlines and the flow field. To date, models both of ideal coronal holes and of specific observed coronal holes indicate that flow velocities above 100 km s+1, and temperatures of perhaps 2 × 106K are possible at 2R ⊙ heliocentric distance, where densities of 2 × 105 cm+3 have been reported. These velocities are at, or just above the sound speed, although still sub-Alfvenic. There is also general agreement among models of large polar holes that conversion of mechanical wave energy flux into solar wind kinetic energy is occurring in the 2R ⊙ to 5R ⊙ range, perhaps occurs even further outwards, and that the magnitude and extent of this energy deposition depends on the size and on the geometrical divergence of the hole. However, each model exhibits distinct weaknesses counteracted only by the complimentary nature of the various types of models. Models in class (i) are simply not global representations, but are tractable when dealing with complex forms of the energy equation or with several ion species. Class (ii) models lack any geometrical information beyond the ad hoc assumption of known streamline geometry, but have the same advantages as those in class (i). Class (iii) models cannot determine streamline geometry within a hole and do not extend further from the Sun than the available data — although they place important constraints on models in the other classes. Class (iv) models are limited to simple forms of the energy equation and/or to quasi-radial flow, but are the only models producing self-consistent streamline geometries through inclusion of transverse magnetic stresses in the momentum equation. Most limitations in coronal hole flow models can be eliminated by using known numerical techniques to combine models in classes (i), (ii), and (iv). This would allow detailed models of coronal holes and corresponding interplanetary conditions to be developed for specific time periods, at the cost of flexibility and possibly also general conceptual understanding. Nevertheless, the concept of a coronal hole is now reasonably well established, and acceptable modelling approaches are rapidly filling the literature. It can be anticipated that the evolution of these models, together with present and future observations, will bring us much nearer to understanding coronal energetics and dynamics.

The solar wind-magnetosphere dynamo and the magnetospheric substorm

In a quiet condition, the solar wind kinetic energy is converted into electrical energy. A small part of this energy is dissipated as heat energy in the polar ionosphere. We identify at least three types of magnetospheric disturbances which are not associated with an increase of the heat production and call them reversible disturbances, while the magnetospheric substorm is an irreversible disturbance which is associated with a large increase of the heat production. The magnetosphere appears to have an inherent internal instability by which a large amount of heat energy is sporadically produced in the polar upper atmosphere at the expense of the magnetic energy in the magnetotail. A positive feed-back process may be responsible for the growth of the instability and for the expansive phase, while the recovery phase sets in when some process begins to suppress the positive feed-back process.

On the cause of geomagnetic bays

The force between the Earth and magnetospheric tail is assumed balanced by a tanential stress on the solar wind. The stress acting against the solar wi wind kinetic energy into magnetic energy stored in the tail. Consequently the tail radius increases and the inner edge of the neutral sheet moves earthward. This one-way development is stopped by a magnetic bay which restores a lower energy state and the process begins again. The estimated time between bays by this mechanism is 2–20 hr. The arguments can be extended to discuss the development of the geomagnetic storm main phase.

A 27-day periodicity in outer zone trapped electron intensities

Data from the polar orbiting satellite 1963 38C show the existence of a 27-day periodicity in the intensities of trapped, ≥280-kev and ≥1.2-Mev electrons for L ≥3.5. This effect is seen for the four sequential solar rotations commencing October 6, 1963. The trapped energetic electron intensities are observed to undergo two large increases every 27 days and are shown to be intimately linked with the passage of the recently observed interplanetary magnetic field sector boundaries. It is also shown that these electron intensity increases are triggered by variations in the parameter β, the ratio of the directed solar wind kinetic energy density to the interplanetary magnetic field energy density. Equivalently, large variations in the Alfvén Mach number MA are shown to be responsible for these trapped electron variations.