Magnetic Activity Index Research Articles

Anomalously low solar and geomagnetic activities in 2007

It has been indicated that 2007 is the year with clearly defined indications typical of a deep solar cycle minimum. In this case this year has no analogs for the entire period of regular solar and geomagnetic parameters concerning very many parameters. The specific behavior of different solar and magnetic activity indices in 2007 has been indicated in comparison with such a behavior of similar characteristics during other periods.

On the reported magnetic precursor of the 1993 Guam earthquake

Using 1‐second magnetometer data recorded 67 km from the epicenter of the 1993 Mw 7.7 Guam earthquake, Hayakawa et al. (1996) and Miyahara et al. (1999) identify anomalous precursory changes in ultra‐low frequency magnetic polarization (the ratio of vertical to horizontal field components). In a check of their results, we compare their data (GAM) with 1‐second data from the Kakioka observatory (KAK) in Japan and the global magnetic activity index Kp. We also examine log books kept by USGS staff working on the Guam magnetic observatory. We find (1) analysis problems with both Hayakawa et al. and Miyahara et al., (2) significant correlation between the GAM, KAK, and Kp data, and (3) an absence of identifiable localized anomalous signals occurring prior to the earthquake. The changes we do find in polarization are part of normal global magnetic activity; they are unrelated to the earthquake.

The persistence of equatorial spread F – an analysis on seasonal, solar activity and geomagnetic activity aspects

Abstract. The persistence (duration) of Equatorial Spread F (ESF), which has significant impact on communication systems, is addressed. Its behavior during different seasons and geomagnetic activity levels under the solar maximum (2001) and minimum (2006) conditions, is reported using the data from the magnetic equatorial location of Trivandrum (8.5° N; 77° E; dip 0.5° N) in India. The study reveals that the persistence of the irregularities can be estimated to a reasonable extent by knowing the post sunset F region vertical drift velocity (Vz) and the magnetic activity index Kp. Any sort of advance information on the possible persistence of the ionospheric irregularities responsible for ESF is important for understanding the scintillation morphology, and the results which form the first step in this direction are presented and discussed.

Hinode Observations of Chromospheric Brightenings in the Ca ii H Line during Small-Scale Flux Emergence Events

\ion{Ca}{2} H emission is a well-known indicator of magnetic activity in the Sun and other stars. It is also viewed as an important signature of chromospheric heating. However, the \ion{Ca}{2} H line has not been used as a diagnostic of magnetic flux emergence from the solar interior. Here we report on Hinode observations of chromospheric \ion{Ca}{2} H brightenings associated with a repeated, small-scale flux emergence event. We describe this process and investigate the evolution of the magnetic flux, G-band brightness, and \ion{Ca}{2} H intensity in the emerging region. Our results suggest that energy is released in the chromosphere as a consequence of interactions between the emerging flux and the pre-existing magnetic field, in agreement with recent 3D numerical simulations.

Uncertainty in magnetic activity indices

Magnetic activity indices are widely used in theoretical studies of solar-terrestrial coupling and space weather prediction. However, the indices suffer from various uncertainties, which limit their application and even mislead to incorrect conclusion. In this paper we analyze three most popular indices, Kp, AE and Dst. Three categories of uncertainties in magnetic indices are discussed: “data uncertainty” originating from inadequate data processing, “station uncertainty” caused by incomplete station covering, and “physical uncertainty” stemming from unclear physical mechanism. A comparison between magnetic disturbances and related indices indicate that the residual Sq will cause an uncertainty of 1–2 in K measurement, the uncertainty in saturated AE is as much as 50%, and the uncertainty in Dst index caused by the partial ring currents is about a half of the partial ring current.

1-8 Å Coronal Background X-Ray Emission and the Associated Indicators of Photospheric Magnetic Activity

Consistent correlations between X-ray background flux (XBF) and sunspot area over three solar cycles suggest a strong relationship between the flux emergence at the locations of strong field regions, sunspots, and the coronal heating processes that ultimately result in coronal X-ray emission. We find that the pattern of variation of sunspot numbers shows noticeable deviations from the pattern depicted by the sunspot areas when compared to XBF. A weight factor, such as Wi, assigned to individual spot groups in R = k(Wigi + n) may provide better estimation of the true strength of solar cycles.

An assessment study of the wavelet-based index of magnetic storm activity (WISA) and its comparison to the Dst index

A wavelet-based index of storm activity (WISA) has been recently developed [Jach, A., Kokoszka, P., Sojka, L., Zhu, L., 2006. Wavelet-based index of magnetic storm activity. Journal of Geophysical Research 111, A09215, doi:10.1029/2006JA011635] to complement the traditional Dst index. The new index can be computed automatically by using the wavelet-based statistical procedure without human intervention on the selection of quiet days and the removal of secular variations. In addition, the WISA is flexible on data stretch and has a higher temporal resolution (1 min), which can provide a better description of the dynamical variations of magnetic storms. In this work, we perform a systematic assessment study on the WISA index. First, we statistically compare the WISA to the Dst for various quiet and disturbed periods and analyze the differences of their spectral features. Then we quantitatively assess the flexibility of the WISA on data stretch and study the effects of varying number of stations on the index. In addition, the ability of the WISA for handling the missing data is also quantitatively assessed. The assessment results show that the hourly averaged WISA index can describe storm activities equally well as the Dst index, but its full automation, high flexibility on data stretch, easiness of using the data from varying number of stations, high temporal resolution, and high tolerance to missing data from individual station can be very valuable and essential for real-time monitoring of the dynamical variations of magnetic storm activities and space weather applications, thus significantly complementing the existing Dst index.

Spatial and time variations in the foF2 (night)/foF2 (day) ratio: Specification of some effects

A series of important aspects of the time and spatial variations in the critical frequency ratio fo(night)/fo(day) considered by one of the authors [Danilov, 2007, 2008] is specified. The list of the Eastern-Hemisphere stations, for which an analysis of the above indicated ratio was performed, is completed. The available stations of the Western Hemisphere are considered. It is shown that the character of the variations in the fo(night)/fo(day) ratio is independent of limitations imposed on the Ap index of magnetic activity for the analyzed days. The consideration of the long-term variations in geomagnetic activity using the annual mean value of the Ap index does not influence the principal conclusions of the study, although in some cases changes statistical characteristics of the obtained behavior of the fo(night)/fo(day) ratio after 1980. A comparison of the results, based on the interpretation of the trends of the fo(night)/fo(day) value with the changes in the stratosphere-ionosphere coupling from the 1980s to the 1990s obtained by the authors earlier, confirms the assumption that there occurs a systematic change in the zonal wind in the upper atmosphere.

Variations in the polar precipitation equatorward boundary during the interaction between the Earth’s magnetosphere and solar wind streams from isolated solar sources

The position of the auroral luminosity equatorward boundary during the interaction between the Earth’s magnetosphere and isolated solar wind streams from different solar sources has been statistically studied based on the ground and satellite observations of auroras. These studies continue the series of the works performed in order to develop the technique for predicting auroras based on the characteristics of the interplanetary medium and auroral disturbances. The dependences of the minimal position of the auroral luminosity equatorward boundary (Φ′) on the values of the azimuthal component of the interplanetary electric field (Ey) and AL indices of magnetic activity, averaged over 6 and 24 h, are presented. The distribution limits for each type of isolated solar wind streams on the Φ′-Ey and Φ′-AL planes have been determined.

The Unpredictability of the Most Energetic Solar Events

Observations over the past two solar cycles show a highly irregular pattern of occurrence for major solar flares, gamma-ray events, and solar energetic particle (SEP) fluences. Such phenomena do not appear to follow the direct indices of solar magnetic activity, such as the sunspot number. I show that this results from the non-Poisson occurrence for the most energetic events. This Letter also points out a particularly striking example of this irregularity in a comparison between the declining phases of the recent two solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the radiated energies of the flares, the associated SEP fluences, and the sunspot areas. These factors suggest that processes in the solar interior involved with the supply of magnetic flux up to the surface of the Sun have strong correlations in space and time, leading to a complex occurrence pattern that is presently unpredictable on timescales longer than active region lifetimes (weeks) and not correlated well with the solar cycle itself.

Contributions of the external field to the observatory annual means and a proposal for their corrections

Abstract In this study we separate, interpret and explain magnetospheric and ionospheric signals present in the observatory annual means. The data from 46 European geomagnetic observatories collected over 42 years (1960–2001) are used. To characterise the various field components, we use predictions from latest magnetic field models. The core field and its secular variation are described by the CM4 model, and the magnetospheric contributions are successfully removed by parameterising the POMME model with the Dst index. We regard the remaining signal as being caused by ionospheric currents. The annual averages of the Sq variation estimated by the CM4 model are subtracted from the residuals. A remaining variation in anti-phase with the magnetic activity index Ap finally can be removed with a function properly scaled by Ap and Dst. We offer an objective procedure to suppress the external field contributions in the annual means to an uncertainty level of ±2 nT. Except for the ionospheric currents, this could be achieved by applying recent magnetic field models, which shows that the quality of present day models is sufficient to correct observatory data for average external field contributions. Understanding the signal contained in the annual means is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.

Recordings of geomagnetically induced currents and a nowcasting service of the Finnish natural gas pipeline system

The geomagnetically induced currents (GIC) in the Finnish natural gas pipeline have been measured at one site in southern Finland since November 1998. We give an overview of this long and uniform time series covering one sunspot maximum. We also briefly discuss the conventional magnetic activity indices K and Ak, and, additionally, the range of the magnetic field and of its time derivative, as indicators of the GIC activity. The measured GIC data formed the basis of the development of a nowcasting service (GIC Now!), which was implemented within the European Space Agency Space Weather Applications Pilot Project. The service provides a near‐real‐time view of the geomagnetic activity level and nowcast estimates of GIC and pipe‐to‐soil voltages in the Finnish natural gas pipeline system. We describe the operation and validation of this service in this paper.

Wavelet‐based index of magnetic storm activity

A wavelet‐based method of computing an index of storm activity is put forward. The new index can be computed automatically using statistical procedures and does not require selecting quiet days and removing the secular component by polynomial fitting. This 1‐min index is designed to facilitate the study of the fine structure of geomagnetic storm events and requires only the most recent magnetogram records, e.g., the 2 months including the storm event of interest. It can thus be computed over a moving window as soon as new magnetogram records become available. Averaged over 1‐hour periods, it is practically indistinguishable from the standard Dst index.

Negative Refractive Index of Meta-materials at Optical Frequencies

ABSTRACTScaling the performance of metamaterials to obtain negative refractive index at optical frequencies has been of great interest. One of the great barriers to the scaling is that real currents cannot be driven at very high frequencies and one is more dependent on displacement currents to generate negative magnetic permeability. Moreover to keep the dimensions of the metamaterials physically accessible, the structural lengthscales of the metamaterials begin approach the wavelength of the radiation in free space and homogenisation is often questionable. Here we will show that metamaterials such as Split ring resonators in these high frequency limits exhibit complex behaviour. Magnetic activity and Negative refractive index behaviour can, indeed, be obtained at optical frequencies but will need to be interpreted very carefully. The plasmonic nature of the metallic system and excitation needs to be considered in detail.

The occurrence of polar cap patches in TEC fluctuations detected using GPS measurements in southern hemisphere

GPS technique is widely used to study the global structure and dynamics of the ionosphere. This paper presents data on the occurrence of large-scale ionospheric irregularities observed at Antarctic stations McMurdo-MCM4 (−78N, 167E, Φ = 80°), Casey-CAS1 (−66N, 110E, Φ = 80°), Mawson-MAW1 (−68N, 63E, Φ = 71°), and Davis-DAV1 (−69N, 78E, Φ = 75°) in 2001. The irregularities caused strong Total Electron Content fluctuations. The intensity of TEC fluctuations was estimated with the ROT parameter and rate of TEC index (ROTI) expressed in TECU/min. Storm-time development of TEC fluctuations are presented for two geomagnetic active periods: June and September 2001. Data on temporal variations in TEC along individual satellites passes were also analyzed. Polar patches were associated with deep TEC fluctuations as the TEC enhancements were 2–10 times larger than the background, whereas the TEC increased by 10–50 TECU in about 5–10 min. The duration of such structures is 10–20 min. At 4–5 h interval, 3–5 strong and some weak patches were detected. The activity of patches depends to a low degree only on the magnetic activity index Kp. The intensity of TEC fluctuations increases during substorm activity. Diurnal, seasonal dependence, and longitudinal features of the occurrence of polar patches are also presented in the report.

Why Kp is such a good measure of magnetospheric convection

The 3‐hour magnetic activity index, Kp, is widely used for measuring the level of magnetospheric activity, and many magnetospheric properties are known to correlate with it. The common denominator for these different properties is the strength of the magnetospheric convection electric field, the large‐scale electric field imposed across the magnetosphere by the flow of the magnetized solar wind past the Earth. While the relationship between Kp and the global convection field has long been known, the question of why the relationship exists has apparently not been addressed. In this report, it is proposed that because Kp is derived from magnetic variations obtained at subauroral stations, it is extremely sensitive to the latitudinal distance to the equatorial edge of the auroral region, where the principal causative currents flow. Since the auroral region maps to the plasma sheet in the magnetosphere, motion of the inner edge of the plasma sheet, which is determined by the strength of the convection field, causes significant changes in Kp. Thus, through its dependence on the latitude of the auroral current region, Kp can be viewed as a direct monitor of the strength of magnetospheric convection, explaining the success of previous Kp‐dependent models of the global electric field.

Historical Space Climate Data from Finland: Compilation and Analysis

We have compiled archived geomagnetic observations from the Helsinki magnetic observatory as well as visual sightings of auroral occurrence in Finland. The magnetic database comprises about 2 000 000 observations of H- and D-components measured during 1844–1909 with time resolution of 10 min to 1 h. In addition, magnetic observations carried out in the First and Second Polar Years in Finland have been recompiled. Magnetic activity indices (three-hour K-and daily Ak-figures) have been derived from the magnetic observations. Comparisons between the Finnish indices and simultaneous global aa-index (starting in 1868) show a good mutual correlation. The Helsinki activity index series can be used as a (pseudo) extension of the aa-index series for about two solar cycles 1844d –1868. On the annual level the correlation coefficient is about 0.9 during the overlapped time interval 1868–1897. The auroral database consists of about 20 000 single observations observed in Finland since the year 1748. The database of visual auroras has been completed by auroral occurrence (AO) index data derived from the Finnish all-sky camera recordings during 1973 –1997 at several sites in Lapland. The AO-index reveals both spatial and temporal variations of auroras from diurnal to solar cycle time scales in different space weather conditions.

High-latitude ionospheric currents during very quiet times: their characteristics and predictability

Abstract. CHAMP passes the geographic poles at a distance of 2.7° in latitude, thus providing a large number of magnetic readings of the dynamic auroral regions. The data of these numerous overflights were used for a detailed statistical study on the level of activity. A large number of tracks with very low rms of the residuals between the scalar field measurements and a high degree field model were singled out over both the northern and southern polar regions, independently. Low rms values indicate best model fits and are therefore regarded as a measure of low activity, although we are aware that this indicator also has its limitations. The occurrence of quiet periods is strongly controlled by the solar zenith angle at the geomagnetic poles, indicating the importance of the ionospheric conductivity. During the dark polar season, about 30% of the passes can be qualified as quiet. The commonly used magnetic activity indices turn out not to be a reliable measure for the activity state in the polar region. Least suitable is the Dst index, followed by the Kp. Slightly better results are obtained with the PC and the IMAGE-AE indices. The latter is rather effective within a time sector of ±4 hours of magnetic local time around the IMAGE array. The orientation of the interplanetary magnetic field (IMF) is an important controlling factor for the activity. This is also supported by the prevailing FAC distribution during quiet times, which resembles the typical NBZ (northward Bz) pattern. In a superposed epoch analysis we show that the merging electric field is a suitable geoeffective solar wind parameter. Based on the size of this electric field and the solar zenith angle at the geomagnetic poles, a prediction method for quiet auroral region periods is proposed. This may, among others, be useful for the data selection in main field modelling approaches.

Imperfect K Index Satisfactory For Electric Power Industry

The 2004 Space Weather article, “Does the electrical power industry need better storm warnings?” is a good description of the argument that the geomagnetic scale known as the K index is inadequate for the power industry. While the K index is not ideal, what we need is not yet practical. For now at least, the K index is one of the most easily understood, best researched, and applicable indices of magnetic activity for the power industry.

Results of the Helsinki magnetic observatory 1844-1912

Abstract. The geomagnetic field declination (D) and horizontal component (H) were observed visually at the Helsinki magnetic observatory between 1844–1912. About 2.0 million single observations of the magnetic components are available. The observing equipment and observation methods were the same for almost 70 years. The Helsinki data series is thus rather homogeneous and suitable for magnetic field analysis of both internal and external origin for about five sunspot cycles (sunspot cycles 9–13). Due to disturbances from nearby electric tramway traffic, most of the observations after 1897 are very noisy and unreliable for magnetic activity studies. Observations of D (1844–1897) have been converted into an absolute scale but H refers to variation values only. Observations of D have been previously analyzed and published for the time interval 1844–1880. In this paper we present new results of D for 1881–1897 and H for 1844–1897. The annual rate of the secular variation of D has been rather stable between 1844–1909, showing a mean eastward increase of +0.11°/year, which is about twice as large as the mean secular variation rate for the past 50 years at the same latitude in Finland. Around 1875 there was a sudden change in the secular variation rate resembling the famous jerk of 1970. Magnetic activity indices (K, Ak) for 1844–1897 were calculated from hourly values of D- and D-series separately using the IAGA K-index algorithm (the FMI-method). Comparisons with other relevant activity series from other sources (aa, u, RI, C9, auroral occurrence rate) show that the Helsinki index series gives an important contribution to the index family. By extending the Mayaud's aa-index series with Helsinki Ak-values (1844–1868), it is possible to reconstruct a (pseudo) aa-series that covers almost 160 years. Magnetic activity (space weather) was appreciably greater during the first three cycles (9–11) than during the two last ones (12–13). The largest magnetic storm occurred in September 1859. Other stormy periods were in 1847, 1852, and 1870–1872. Mean magnetic activity remained exceptionally low in years 1875–1881 and 1887–1891. In an FFT-analysis of the activity index series, the solar cycle, annual, semiannual, solar rotation and half solar rotation spectral lines are well established.Key words. Geomagnetism and paleomagnetism (time variations, diurnal to secular) – History of geophysics (planetology) – Magnetospheric physics (solar wind-magnetosphere interactions)7