The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.     

About the relationship between auroral electrojets and ring currents

The relationship between the storm-time ring current and the auroral electrojets is investigated using IMAGE magnetometer data, D_St and H-SYM, and solar wind data. Statistical results as well as the investigation of single events show that the auroral electrojets occur also during nonstorm conditions without storm-time ring current development and even during the storm recovery phase of increasing D_St. A close correlation between electrojet intensity and ring current intensity was not found. Though the eastward electrojet moves equatorward during the storm main phase there is no unequivocal relationship between the movement of the westward electrojet and the ring current development. All these results suggest that the auroral electrojets and the ring current develop more or less independently of each other.     

Simultaneous observations at different altitudes of ionospheric backscatter in the eastward electrojet

A common feature of evening near-range ionospheric backscatter in the CUTLASS Iceland radar field of view is two parallel, approximately L-shell-aligned regions of westward flow which are attributed to irregularities in the auroral eastward electrojet region of the ionosphere. These backscatter channels are separated by approximately 100–200 km in range. The orientation of the CUTLASS Iceland radar beams and the zonally aligned nature of the flow allows an approximate determination of flow angle to be made without the necessity of bistatic measurements. The two flow channels have different azimuthal variations in flow velocity and spectral width. The nearer of the two regions has two distinct spectral signatures. The eastern beams detect spectra with velocities which saturate at or near the ion-acoustic speed, and have low spectral widths (less than 100ms^–1), while the western beams detect lower velocities and higher spectral widths (above 200ms^–1). The more distant of the two channels has only one spectral signature with velocities above the ionacoustic speed and high spectral widths. The spectral characteristics of the backscatter are consistent with E-region scatter in the nearer channel and upper-E-region or F-region scatter in the further channel. Temporal variations in the characteristics of both channels support current theories of E-region turbulent heating and previous observations of velocity-dependent backscatter cross-section. In future, observations of this nature will provide a powerful tool for the investigation of simultaneous E- and F-region irregularity generation under similar (nearly co-located or magnetically conjugate) electric field conditions.     

Monitoring ionospheric response to auroral electrojet activity from sub-auroral to equatorial latitudes in the East Asian-Australian longitudinal sector over a solar cycle (1978–1986)

Large auroral and ionospheric databases, covering a solar cycle (1978–1986), were used to obtain a comprehensive evaluation of the auroral electrojet effect (as inferred from the auroral AE-index) on the ionospheric response in both hemispheres from sub-auroral to equatorial latitudes. The study was limited to the East Asian-Australian longitudinal sector where data are available from a chain of nine latitudinally displaced stations. Enhancement in the standard ionospheric parameter, the virtual height of the F-region (Δh′F) recorded by vertical-incidence ionosondes, was used to trace the ionospheric disturbance.Unlike the previous studies of this type, the total magnetic and ionospheric data, in hourly intervals, were used to derive the correlation coefficient r between two intrinsically different parameters: Δh′F and AE-index for the local nighttime (20–06 LT or 10–20 UT). A suitable averaging and smoothing technique was applied to the data to enhance the correlation trend between these parameters. It is evident that the height fluctuations of sub-auroral ionosphere (for stations: Yakutsk in Siberia and Hobart and Canberra in Australia) closely resemble the auroral electrojet surges, inferred from the AE-index over the solar cycle. The linear coefficient r is highly significant, being close to 0.6 for most of the time; during the years of maximum auroral activity (1981–1983) r approached 0.8. The consistently high correlation r, regardless of the season, applies only to the most poleward station used in this study, Yakutsk. The sub-auroral stations (Hobart and Canberra) positioned further equatorwards show a strong decline in the correlation coefficient r during the local summer but have high r during winter and the equinoxes. There is a general decline in r towards lower latitudes, suggesting that the response to auroral substorms is on the whole diminishing with the distance from the auroral source to the equator. There appears to be an anomalous increase in r as observed around 10° invariant latitude.These findings appear to be the first long-term proof of the symmetry of the ionospheric responses to auroral substorm activity in the northern and southern auroral ovals which is an important contribution to space climatology. It is suggested that the aurorally generated acoustic gravity waves (AGWs), manifested in the global ionosphere as large scale travelling ionospheric disturbances (LSTIDs), may contribute to the observed auroral-ionospheric phenomena.     

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

On August 21st 1998, a sharp southward turning of the IMF, following on from a 20 h period of northward directed magnetic field, resulted in an isolated substorm over northern Scandinavia and Svalbard. A combination of high time resolution and large spatial scale measurements from an array of coherent scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive substorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement of the electron temperature and UV aurora. This poleward expanding auroral structure moves initially at 0.9 km s^-1 poleward, finally reaching a latitude of 72.5°. The optical signature expands rapidly westwards at ~6 km s^-1, whilst the eastward edge also expands eastward at ~0.6 km s^-1. Typical flows of 600 m s^-1 and conductances of 2 S were measured before the auroral activation, which rapidly changed to ~100 m s^-1 and 10–20 S respectively at activation. The initial flow response to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, imposed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of low-electric field, but also low conductivity poleward of the high-conductivity region. Rapid flows are observed at the edge of the high-conductivity region, and subsequently the high flow region develops, flowing around the expanding auroral feature in a direction determined by the flow pattern prevailing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2° further poleward than the UV auroral signature associated with it.     

Optical effects in the aurora caused by ionospheric HF heating

Ionospheric heating experiments were done by the EISCAT Heater in Tromsø on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojärvi about 100 km to the south-east of Tromsø. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O₂⁺ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.     

亚暴期间极光电集流带的变化

极光活动加剧和极光电集流增强是磁层-电离层能量耦合的两种重要表现形式,它们同为磁层带电粒子向电离层沉降的结果,但是变化规律却非常不同.本文用地基磁场资料,反演极区等效电流体系,研究地磁平静期和扰动期极光电集流带的运动特点.研究表明,Harang间断把极光电集流带分为两段：下午—黄昏段的东向电集流带较弱,而晨侧和子夜—凌晨段的西向电集流带较强.在亚暴膨胀相,随着AE指数增大,整个极光卵向赤道扩展,而极光电集流带却表现出分段差异的特点：下午—黄昏东向电集流带向低纬移动,晨侧西向电集流带也向赤道移动,而子夜—凌晨西向电集流带则向极移动.电动力学分析表明,在不同地方时段,控制电流的主要因素不同,因而,电流及其磁扰有不同的特点：下午—黄昏东向电集流和晨侧西向电集流组成了DP2电流体系,主要受控于磁层对流电场,反映了“驱动过程”的行为;而子夜—凌晨西向电集流是DP1电流体系的基本部分,主要受控于电导率,反映了“卸载过程”的特点.     

Quantitative analysis of the relationship between polar ionospheric currents and auroral electrojet indices

Magnetic storms and substorms are principalprocesses of energy transition from the solar wind intothe magnetosphere-ionosphere system and dissipationin the system. They are also important events whichthe space physics study and space weather predictionhave been focused on. Magnetic storms are describedby means of the magnetic index Dst, which is calcu-lated using the magnetic disturbances of horizontalcomponent recorded at 5 low-latitudinal stations, rep-resenting approximately the symmetric r…     

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere

A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s^–1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.     

Auroral electron precipitation when the trapped radiation region collapsed during the main phase of the geomagnetic storm of May 15, 2005

The simultaneous measurements of the boundary of the trapped radiation region, where auroral electrons precipitate, on the Meteor-3M satellite (the circular polar orbit at an altitude of ∼1000 km) and the westward electrojet dynamics during the main phase of a strong (Dst = −263 nT) magnetic storm that occurred on May 15, 2005, are analyzed. At the end of the first hour of the storm main phase, the nightside boundary of the trapped radiation region and the peak of the precipitating electron fluxes with a energies of ∼1 keV shifted toward the Earth to L ∼ 3. The westward electrojet center approached the same L shell. Near the boundary of the trapped radiation region, the auroral electron spectrum had the shape of typical inverted V. The differential spectrum maximum shifted to an energy of ∼100 eV, when the latitude decreased by ∼1°. The nightside boundary of the trapped radiation region, the electron precipitation equatorward boundary, and the westward electrojet center are compared with the known empirical dependences of the position of these structures on the Dst variation amplitude.     

Non-magnetic aspect sensitive auroral echoes from the lower E region observed at 50 MHz

Backscatter from E-region irregularities was observed at aspect angles close to 90° (almost parallel to the direction of the magnetic field) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric fields and increased E-region electron temperatures simultaneously measured with the incoherent scatter facility EISCAT proved that the Farley-Buneman plasma instability was excited. In addition, strong particle precipitation was present as inferred from EISCAT electron densities indicating that the gradient drift instability may have been active, too. Backscatter at such large aspect angles was not expected and has not been observed before. The characteristics of the observed echoes, however, are in many aspects completely different from usual auroral radar results: the Doppler velocities are only of the order of 10 m/s, the half-width of the spectra is around 5 m/s, the echoes originate at altitudes well below 100 km, and they seem to be not aspect-sensitive with respect to the magnetic field direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.     

Ionospheric response to variable electric fields in small-scale auroral structures

High time and space resolution optical and radar measurements have revealed the influence of electric fields on E-region electron density profiles in small-scale auroral structures. Large electric fields are present adjacent to auroral filaments produced by monoenergetic electron fluxes. The ionisation profiles measured within and beside the auroral filaments show the effects of plasma convection due to electric fields as well as the consequences of the response time to large and dynamic fluxes of energetic electrons. Without high-resolution optical measurements, the interpretation of the radar data is limited.     

Seasonal and solar cycle variations of the auroral electrojet indices

Using hourly mean auroral electrojet indices for the past 20 years, we examine the seasonal and solar cycle variations of the AU and AL indices as well as the smaller time-scale fluctuations in these indices. The AU and AL indices maximize during summer and equinoctial months, respectively. By removing the effects of the solar conductance from the AU index, it is found that the electric field contribution to the AU index exhibits the same semiannual variation pattern as the AL index, indicating that the semiannual magnetic variations are controlled by the electric field. Since the auroral electrojets are mostly Hall currents flowing in the east–west direction, the fluctuations of the auroral electrojet indices can be interpreted in terms of fluctuations in the north–south component of the electric field and the Hall conductance. The AU fluctuation is largely due to that of the electric field, while the AL fluctuation is attributed to both the electric field and Hall conductance with their contributions being comparable. The high fluctuation of AL compared to that of AU is attributed to particle precipitation associated with substorm activity. However, the fluctuations of the electric field and conductance do not show any noticeable seasonal dependence. The variation pattern of the yearly mean AL index follows the mirror image of the AU index during the past 20 years, indicating that the absolute values of the two indices are proportional to each other. This suggests again that the electric field is the main modulator of magnetic disturbance. On the other hand, they show a tendency to become higher during the declining phase of the solar cycle. This is the same variation pattern confirmed from the aa index. However, the fluctuations of the electric field and the Hall conductance do not show any apparent dependence on the solar cycle.     

Modulation of whistlers

Analysis of the experimental data obtained at Paratunka observatory (53.02° N, 158.65° E; L = 2.3) has revealed a nonstandard form of whistlers involving spectral lines that are symmetric with respect to the whistler. We have shown that this form is most likely due to the amplitude modulation of whistlers by electromagnetic pulses with a length of around 1 s and carrier frequency of around 1.1 kHz. We have suggested that these pulses could be emitted by the auroral electrojet modified by heating radiation from the HAARP facility (62.30° N, 145.30° W; L > 4.2).     

Global geomagnetic and auroral response to variations in the solar wind dynamic pressure on April 1, 1997

The geomagnetic and auroral response to the variations in the solar wind dynamic pressure (Pd) are investigated in the periods of positive values of the IMF B _z component. It is shown that the growth of Pd results in the intensification of luminosity along the auroral oval and in the poleward expansion of the poleward boundary of luminosity (PBL) in the nightside part of the oval by ～7° in latitude at a velocity of ～0.5 km/s and is accompanied by an enhancement of the DP2-type current system. A decrease in Pd, accompanied by an abrupt reversal of the IMF B _y polarity from positive to negative, results in an enhancement of the westward electrojet and in a poleward shift of PBL and electrojet center. The conclusion has been made that the available three types of auroral response to Pd variations differ in the azimuthal velocity of the luminosity region or particle precipitation along the auroral oval: V ₁ ～ 30–40 km/s, V ₂ ～ 10, and V ₃ ～ 1 km/s.     

Ionospheric signatures of magnetospheric boundaries in the post-noon sector

Spatial structures in ionospheric electron density revealed in a tomographic image have been identified with auroral forms and related to their sources in precipitating particles observed by DMSP satellites. The observations of plasma enhancements relate to discrete auroral arcs seen in the post-noon sector, identified by both red- and green-line emissions measured by a meridional scanning photometer. The features lie within a very narrow latitudinal band on L-shells where the satellite detectors observed electron precipitation classified as from the boundary plasma sheet (BPS). The harder particles are identified with an E-region structure, while further north the precipitation is softer, resulting in a localised F-layer blob and 630.0 nm emissions. A steep gradient in plasma density represent a signature in the ionosphere of the central plasma sheet (CPS)/BPS boundary. A transition to a less-structured F-layer is found on crossing the convection reversal boundary.On leave from Artic Geophysics, University Course on Svalbard (UNIS), N-9170 Longyearbyen, Norway     

The planetary scale distribution of telluric currents and the effect of the equatorial electrojet: An investigation by canonical GDS

The planetary scale distribution of electrical currents in the Earth is still largely unknown. The role of the oceans for long period (hours to days) inducing electromagnetic fieldsB _e of external origin has been investigated by several authors, while the role of telluric current channelling, from the planetary viewpoint, is still far from a satisfactory understanding. Canonical geomagnetic depth sounding (GDS) analysis can yield locally a direction parallel to the strike of a telluric current density flowing in a region around the recording site and which also has the property of being the most relevant source for the internal origin fieldB _i observed at the given site at the given frequency. The use of such local information from 64 geomagnetic observatories is discussed here in a study to infer evidences of (a) the role of the polarization properties ofB _e and (b) the role of the telluric current channelling within conductivity anomalies relevant to the planetary scale circuitry. The results show clear evidence of the influence of the equatorial electrojet on the polarization ofB _e in a latitudinal band between ±(15°–20°) latitude. There is also evidence that theB _e associated with the equatorial electrojet produces telluric currents which flow at a much shallower depth than the skin depth to be expected in the case of a plane Earth. This implies that the Parkinson planes in these regions reflect the conductivity structure underground more than the polarization ofB _e due to the equatorial electrojet. Further, it clearly appears that some regular planetary scale pattern of telluric currents plays a more significant role than current channelling within some conductivity anomalies of fixed strike close to some geomagnetic observatories. Finally, the number of observatories used in this study appears to be insufficient to deduce any information concerning a seasonal evolution of the telluric current pattern on a planetary scale.     

Analysis of latitudinal distribution of Pi2 geomagnetic pulsations using the generalized variance method

The spatial dynamics of bursts of geomagnetic Pi2-type pulsations during a typical event of a magnetospheric substorm (April 13, 2010) drifting to the pole was investigated using the method of generalized variance characterizing the integral time increment of the total horizontal amplitude of the wave at a given point in the selected time interval. The digital data of Scandinavian profile observations from IMAGE magnetometers with 10-second sampling and data of the INTERMAGNET project observations at the equatorial, middle-latitude and subauroral latitudes with a 1-second sampling were used in the analysis. It was shown that Pi2 pulsation bursts in a frequency band of 8–20 mHz appear simultaneously on a global scale: from the polar to equatorial latitudes with maximum amplitudes at latitudes of the maximum intensity of the auroral electrojet and with a maximum amplitude of geomagnetic pulsations Pi3 within a band of 1.5–6 mHz. The first (left-polarized) intensive Pi2 burst appeared at auroral latitudes several minutes after breakup, while the second (right-polarized) burst occurred 15 min after breakup but at higher (polar) latitudes where the substorm had displaced by that time. The direction of wave-polarization vector rotation was opposite for auroral and subauroral latitudes, but it was identical at the equator and in the subauroral zone. The pulsation amplitude at the equator was maximal in the night sector.     

Extension of MF radar tidal measurements to E-region heights (95-125 km): Saskatoon (52○N, 107○W), Canada

Efforts have been made to extend the MF radar tidal profiles to E-region heights. The totally reflected MF radar echoes from E-region heights during daytime are known to be group-retarded and the corresponding wind and tidal data will have associated height discrepancies. The estimation of the E-region real heights (Namboothiri et al., 1993), and the elimination of the data for which the group retardation is significant, are selected as the basic criteria to extend the tidal profiles to 100–125 km. The analysis of the quiet (A_p<19) days of the winter and summer seasons of 1988/89 shows that the tidal propagation continues to higher altitudes with some changes in their pattern, e.g. longer wavelengths, compared to that in the lower altitudes. Comparison with the model profiles shows some resemblance and some disagreements. The reliability of the MF radar tidal measurements of E-region heights and the propagation of tides in this region have been discussed in the light of existing theories and other experimental observations. It is concluded that, based on the initial studies with UHF and MF systems and within the limits of the available theories, the information on tides presented here for the 100–125 km region using the MF radar observations is useful. Suggestions for future work in this direction are also given.     

Ring current and auroral electrojets in connection with interplanetary medium parameters during magnetic storm

The relationship between the auroral electrojet indices (AE) and the ring current magnetic field (DR) was investigated by observations obtained during the magnetic storm on 1–3 April 1973. During the storm main phase the DR development is accompanied by a shift of the auroral electrojets toward the equator. As a result, the standard AE indices calculated on the basis of data from auroral observatories was substantially lower than the real values (AE’). To determine AE’ during the course of a storm main phase data from subauroral magnetic observatories should be used. It is shown that the intensity of the indices (AE’) which take into account the shift of the electrojets is increased substantially relative to the standard indices during the storm main phase. AE’ values are closely correlated with geoeffective solar wind parameters. A high correlation was obtained between AE’ and the energy flux into the ring current during the storm main phase. Analysis of magnetic field variations during intervals with intense southward IMF components demonstrates a decrease of the saturation effect of auroral electrojet currents if subauroral stations magnetic field variations are taken into account. This applies both to case studies and statistical data. The dynamics of the electrojets in connection with the development of the ring current and of magnetospheric substorms can be described by the presence (absence) of saturation for minimum (maximum) AE index values during a 1-h interval. The ring current magnetic field asymmetry (ASY) was calculated as the difference between the maximum and minimum field values along a parallel of latitude at low latitudes. The ASY value is closely correlated with geoeffective solar wind parameters and simultaneously is a more sensitive indicator of IMF B_z variations than the symmetric ring current. ASY increases (decreases) faster during the main phase (the recovery phase) than DR. The magnetic field decay at low latitudes in the recovery phase occurs faster in the afternoon sector than at dusk.