Byung-Ho Ahn

Universal time variations of the auroral electrojet indices

Using the hourly mean AE indices for the past 20 years, amounting to a total of 175,296 hours, we examine how the longitudinal station gaps of the present AE network affect the ability to monitor accurately the auroral electrojets. The latitudinal shift of the auroral electrojet location with magnetic activity also affects the reliability of the AE indices. These combined effects would result in pronounced universal time (UT) variations of the AE indices. By counting the number of occurrences recorded during the given ranges of activity, say every 100 and 200 nT for the AU and AL indices, respectively, for each hour of universal time, the UT variations of the two indices are examined separately. The result demonstrates clearly that they are strongly dependent upon UT. Furthermore, it is noted that the equatorward expansion of the auroral electrojets is more responsible for the UT variation than are the longitudinal station gaps. For the range of the magnetic activity levels examined in this study, i.e., 0 to 500 nT and 0 to -1000 nT for the AU and AL indices, the centers of the eastward and westward electrojets seem to be located within the latitudinal ranges of 71°-65° and 68°-62°, respectively. The seasonal change of ionospheric conductance also contributes to the UT variation, particularly that of the AL index. While maintaining a similar variation pattern, the amplitude of the variation increases during winter and decreases during summer. It indicates that the UT variation of the AL index is more serious during winter than summer. With more AE stations being located within the former range than the latter, it is easily understood why the AL index is more strongly dependent on UT than is the AU index. Considering such a latitudinal distribution, it is highly probable that the present AL indices often underestimate disturbed conditions during specific universal time intervals, particularly 0200-0800 UT.

Climatological characteristics of the auroral ionosphere in terms of electric field and ionospheric conductance

The contributions of the north-south component of the electric field and the Hall conductance to the auroral electrojet are examined separately. For this purpose, 52 days of measurements from the Chatanika incoherent scatter radar, which was located near one of the standard AE stations, College, are utilized. A number of interesting characteristics of the auroral electrojet system and auroral electrojet indices are noted: (1) The electric field distribution along the auroral region is roughly symmetric with respect to the 1100-2300 magnetic local time meridian. (2) The electric field, particularly the southward component, becomes a dominant feature along auroral latitudes with increasing magnetic activity. (3) The Hall conductance distribution in the postnoon sector is mainly determined by the Sun, thus making the eastward electrojet and the AU index dependent upon season. On the other hand, the Hall conductance associated with the major part of the westward electrojet in the midnight-postmidnight sector is controlled by precipitating electrons. (4) Since the Hall conductance of solar origin in the postnoon sector can be estimated, it would be possible to monitor electric field enhancements contributing to the eastward electrojet. By assuming the same electric field, except for the sign being applied to the westward electrojet, the AL index can be used to estimate the contribution of the Hall conductance associated with particle precipitation. This is a clear indication that the two indices, AU and AL, are governed by different physical processes. Thus it is recommended to use the two indices separately, rather than the combined AE, in monitoring the auroral electrojet system. (5) The Harang discontinuity seems to be a boundary separating the region of precipitating energetic particles on its northeast side from that of soft particles on its southwest side.

Substorm changes of the electrodynamic quantities in the polar ionosphere: CDAW 9

On the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from Viking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 1986, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Scandinavia and Russia chains of magnetometers were located in the premidnight-midnight sector during this interval and the estimated conductivity distribution is instantaneous, our data set provides us with a unique opportunity to examine some long-standing problems associated with the substorm expansion onset. Several important findings of this study are summarized as follows: (1) Before the expansion onset of the substorm, intensifications of ionospheric currents or the cross-polar cap potential are very weak in this particular example. Both quantities begin to increase notably only with the initiation of the substorm expansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase, while its equatorward half is occupied by a weak eastward electrojet. (3) The Joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset. During the expansion phase of the substorm, however, Joule heating shows a marked intensification, but the latter increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm times. (4) The Hall currents are not completely divergence-free. The corresponding field-aligned currents show highly localized structures during the maximum epoch of the substorm, with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector. This is indirect evidence that the so-called imperfect Cowling channel is effective behind the westward traveling surge.

Ionospheric F2-Layer Semi-Annual Variation in Middle Latitude by Solar Activity

E-mail: yskwak@kasi.re.kr Tel: +82-42-865-2039 Fax: +82-42-865-2020This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://cre-ativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Dst Prediction Based on Solar Wind Parameters

We reevaluate the Burton equation (Burton et al. 1975) of predicting Dst index using high quality hourly solar wind data supplied by the ACE satellite for the period from 1998 to 2006. Sixty magnetic storms with monotonously decreasing main phase are selected. In order to determine the injection term (Q) and the decay time (?) of the equation, we examine the relationships between Dst ⁄ and V Bs, ¢Dst ⁄ and V Bs, and ¢Dst ⁄ and Dst ⁄ during the magnetic storms. For this analysis, we take into account one hour of the propagation time from the ACE satellite to the magnetopause, and a half hour of the response time of the magnetosphere/ring current

Seasonal and Latitudinal Variations of the F2-Layer during Magnetic Storms

Department of Earth Science Education, Kyungpook National University, Daegu 702-701, KoreaTo identify seasonal and latitudinal variations of F2 layer during magnetic storm, we examine the change of daily averages of foF2 observed at Kokubunji and Hobart during high (2000~2002) and low (2006~2008) solar activity intervals. It is found that geomagnetic activity has a different effect on the ionospheric F2-layer electron density variation for different seasons and different latitudes. We, thus, investigate how the change of geomagnetic activity affects the ionospheric F2-layer electron density with season and latitude. For this purpose, two magnetic storms occurred in equinox (31 March 2001) and solstice (20 November 2003) seasons are selected. Then we investigate foF2, which are observed at Kokubunji, Townsville, Brisbane, Canberra and Hobart, Dst index, Ap index, and AE index for the two magnetic storm periods. These observatories have similar geomagnetic longitude, but have different latitude. Furthermore, we investigate the relation between the foF2 and the [O]/[N

Comments on Some Long-Standing Problems in Storm/Substorm Studies

We identify seven long-standing problems in storm/substorm studies and present our views on them mainly on the basis of earlier literatures, demonstrating that earlier studies should be combined with new results to overcome the difficulties associated with long-standing problems. We introduce a new index AF to monitor the upward field-aligned currents (FAC) and show that it is far better correlated to the Dst index than the AE index.

Contributions of Heating and Forcing to the High-Latitude Lower Thermosphere: Dependence on the Interplanetary Magnetic Field

To better understand the physical processes that maintain the high-latitude lower thermospheric dynamics, we have identified relative contributions of the momentum forcing and the heating to the high-latitude lower thermospheric winds depending on the interplanetary magnetic field (IMF) and altitude. For this study, we performed a term analysis of the potential vorticity equation for the high-latitude neutral wind field in the lower thermosphere during the southern summertime for different IMF conditions, with the aid of the National Center for Atmospheric Research Thermosphere- Ionosphere Electrodynamics General Circulation Model (NCAR-TIEGCM). Difference potential vorticity forcing and heating terms, obtained by subtracting values with zero IMF from those with non-zero IMF, are influenced by the IMF conditions. The difference forcing is more significant for strong IMF B y condition than for strong IMF B z condition. For negative or positive B y conditions, the difference forcings in the polar cap are larger by a factor of about 2 than those in the auroral region. The difference heating is the most significant for negative IMF B z condition, and the difference heat- ings in the auroral region are larger by a factor of about 1.5 than those in the polar cap region. The magnitudes of the difference forcing and heating decrease rapidly with descending altitudes. It is confirmed that the contribution of the forcing to the high-latitude lower thermospheric dynamics is stronger than the contribution of the heating to it. Espe- cially, it is obvious that the contribution of the forcing to the dynamics is much larger in the polar cap region than in the auroral region and at higher altitude than at lower altitude. It is evident that when B z is negative condition the contribu- tion of the forcing is the lowest and the contribution of the heating is the highest among the different IMF conditions.

Analysis of wind vorticity and divergence in the high-latitude lower thermosphere: Dependence on the interplanetary magnetic field (IMF)

To better understand the physical processes that control the high-latitude lower thermospheric dynamics, we analyze the divergence and vorticity of the high-latitude neutral wind field in the lower thermosphere during the southern summertime for different IMF conditions. For this study the National Center for Atmospheric Research Thermosphere-Ionosphere Electrodynamics General Circulation Model (NCAR-TIEG CM) is used. The analysis of the large-scale vorticity and divergence provides basic understanding flow configurations to help elucidate the momentum sources that ulti-mately determine the total wind field in the lower polar thermosphere and provides insight into the relative strengths of the different sources of momentum responsible for driving winds. The mean neutral wind pattern in the high-latitude lower thermosphere is dominated by rotational flow, imparted primarily through the ion drag force, rather than by divergent flow, imparted primarily through Joule and solar heating. The difference vorticity, obtained by subtracting values with zero IMF from those with non-zero IMF, in the high-latitude lower thermosphere is much larger than the difference divergence for all IMF conditions, indicating that a larger response of the thermospheric wind system to enhancement in the momentum input generating the rotational motion with elevated IMF than the corresponding energy input generating the divergent motion. the difference vorticity in the high-latitude lower thermosphere depends on the direction of the IMF. The difference vorticity for negative and positive shows positive and negative, respectively, at higher magnetic latitudes than . For negative , the difference vorticities have positive in the dusk sector and negative in the dawn sector. The difference vorticities for positive have opposite sign. Negative IMF has a stronger effect on the vorticity than does positive .

Response of the Geomagnetic Activity Indices to the Solar Wind Parameters

This study attempts to show how the geomagnetic indices, AU, AL and Dst, respond to the interplanetary parameters, more specifically, the solar wind electric field VBz during southward interplanetary magnetic field (IMF) period. The AU index does not seem to respond linearly to the variation of southward IMF. Only a noticeable correlation between the AU and VBz is shown during summer, when the ionospheric conductivity associated with the solar EUV radiation is high. It is highly likely that the effect of electric field on the eastward electrojet intensification is only noticeable whenever the ionospheric conductivity is significantly enhanced during summer. Thus, one should be very cautious in employing the AU as a convection index during other seasons. The AL index shows a significantly high correlation with VBz regardless of season. Considering that the auroral electrojet is the combined result of electric field and ionospheric conductivity, the intensification of these two quantities seems to occur concurrently during southward IMF period. This suggests that the AL index behaves more like a convection index rather than a substorm index as far as hourly mean AL index is concerned. Contrary to the AU index, the AL index does not register the maximum value during summer for a given level of VBz. It has something to do with the findings that discrete auroras are suppressed in sunlight hemisphere (Newell et al. 1996), thus reducing the ionospheric conductivity during summer. As expected, the Dst index tends to become more negative as VBz gets intensified. However, the Dst index (nT) is less than or equal to 15 VBz (mV/m) + 50 (Bz ). It indicates that VBz determines the lower limit of the storm size, while another factor(s), possibly substorm, seems to get further involved in intensifying storms. Although it has not been examined in this study, the duration of southward IMF would also be a factor to be considered in determining the size of a storm.