Auroral Frequency Research Articles

Evaluation of the 900‐Year European Auroral Records With Extreme Value Theory

AbstractAn explicit proxy of solar activity on the Earth is the auroral displays. The auroral oval can extend to lower latitudes during geomagnetic storms triggered by explosive solar activities. The lower the latitude of auroral oval is, the stronger the solar activity is. Systematic auroral records in Europe (mainly central Europe with geomagnetic latitude lower than 55°) can be dated back to 1000 AD and can be used to manifest solar activity in the past millennium. However, the temporal distribution of the 6,262 auroral records during 1000 and 1900 AD is seriously uneven, with 85.6% of the records appeared after 1700 AD. Here we use the extreme value theory (EVT) to evaluate the effectiveness of characterizing the solar activity with the auroral records before 1700 AD. Due to the inhomogeneity of the auroral records, the EVT has been conducted separately for the data before 1700 and after 1700, finding that the 100‐year auroral frequency is 25.66 [19.67, 33.67] based on the auroral data in 1000–1699, and 283.71 [183.14, 417.58] based on the auroral data in 1700–1900. The predictions of both the 50‐year and 100‐year auroral frequencies before 1700 AD agree well with the solar activity index.

Equatorial auroral records reveal dynamics of the paleo-West Pacific geomagnetic anomaly

Localized regions of low geomagnetic intensity such as the South Atlantic Anomaly allow energetic particles from the Van Allen radiation belt to precipitate into the atmosphere and have been linked to a signature in the form of red aurora-like airglow visible to the naked eye. Smoothed global geomagnetic models predict a low-intensity West Pacific Anomaly (WPA) during the sixteenth to nineteenth centuries characterized by a simple time dependence. Here, we link the WPA to an independent database of equatorial aurorae recorded in Seoul, South Korea. These records show a complex fluctuating behavior in auroral frequency, whose overall trend from 1500 to 1800 AD is consistent with the locally weak geomagnetic field of the WPA, with a minimum at 1650 AD. We propose that the fluctuations in auroral frequency are caused by corresponding and hitherto unknown fluctuations in the regional magnetic intensity with peaks at 1590 and 1720 AD, a time dependence that has been masked by the smoothing inherent in regularized global geomagnetic models. A physical core flow model demonstrates that such behavior requires localized time-dependent upwelling flows in the Earth's core, possibly driven by regional lower-mantle anomalies.

The Dalton Minimum and John Dalton’s Auroral Observations

In addition to the regular Schwabe cycles of approximately 11 y, “prolonged solar activity minima” have been identified through the direct observation of sunspots and aurorae, as well as proxy data of cosmogenic isotopes. Some of these prolonged minima have been regarded as grand solar minima, which are arguably associated with the special state of the solar dynamo and have attracted significant scientific interest. In this paper, we review how these prolonged solar activity minima have been identified. In particular, we focus on the Dalton Minimum, which is named after John Dalton. We review Dalton’s scientific achievements, particularly in geophysics. Special emphasis is placed on his lifelong observations of auroral displays over approximately five decades in Great Britain. Dalton’s observations for the auroral frequency allowed him to notice the scarcity of auroral displays in the early 19th century. We analyze temporal variations in the annual frequency of such displays from a modern perspective. The contemporary geomagnetic positions of Dalton’s observational site make his dataset extremely valuable because his site is located in the sub-auroral zone and is relatively sensitive to minor enhancements in solar eruptions and solar wind streams. His data indicate clear solar cycles in the early 19th century and their significant depression from 1798 to 1824. Additionally, his data reveal a significant spike in auroral frequency in 1797, which chronologically coincides with the “lost cycle” that is arguably located at the late Solar Cycle 4. Therefore, John Dalton’s achievements can still benefit modern science and help us improve our understanding of the Dalton Minimum.

Polarization measurements of unusual cases of medium frequency burst emissions extending below 1.5\xa0MHz

Auroral medium frequency burst (MFB) is a radio emission of natural auroral origin associated with substorm expansion phase and observable at ground level. The emission usually occurs at frequencies above 1500 kHz, but occasionally it extends to a sharp lower cutoff frequency at 1300–1500 kHz depending on the observing site, with a frequency gap below the cutoff and sporadic emission below the gap extending to frequencies as low as 1000 kHz. These low-frequency MFB components lie below the electron gyrofrequency and hence could represent either whistler or LO-modes. Recently, using crossed antennas and a two-channel receiver at Toolik Lake, Alaska, polarization of these low-frequency MFB components was measured for the first time and found to be left-hand. This observation eliminates whistler mode as a possibility and requires the low-frequency components be LO-mode in the ionosphere, which constrains their source location since it requires that the frequency exceeds the L-cutoff frequency. In these occasional events marked by a cutoff and low-frequency MFB components, the latter probably originate at high altitudes (>,800 km) and reach the ground through extraordinary low-density polar cap ionosphere.

Sophus Peter Tromholt: an outstanding pioneer in auroral research

Abstract. The Danish school teacher Sophus Peter Tromholt (1851–1896) was self-taught in physics, astronomy, and auroral sciences. Still, he was one of the brightest auroral researchers of the 19th century. He was the first scientist ever to organize and analyse correlated auroral observations over a wide area (entire Scandinavia) moving away from incomplete localized observations. Tromholt documented the relation between auroras and sunspots and demonstrated the daily, seasonal and solar cycle-related variations in high-latitude auroral occurrence frequencies. Thus, Tromholt was the first ever to deduce from auroral observations the variations associated with what is now known as the auroral oval termed so by Khorosheva (1962) and Feldstein (1963) more than 80 yr later. He made reliable and accurate estimates of the heights of auroras several decades before this important issue was finally settled through Størmer's brilliant photographic technique. In addition to his three major scientific works (Tromholt, 1880a, 1882a, and 1885a), he wrote numerous short science notes and made huge efforts to collect historical auroral observations (Tromholt, 1898). Furthermore, Tromholt wrote a large number of popular science articles in newspapers and journals and made lecture tours all over Scandinavia and Germany, contributing to enhance the public educational level and awareness. He devoted most of his life to auroral research but as a self-taught scientist, he received little acclaim within the contemporary academic scientific society. With his non-academic background, trained at a college of education – not a university – he was never offered a position at a university or a research institution. However, Sophus Tromholt was an outstanding pioneer in auroral research.

Characterization of a Penning discharge for investigation of auroral radio wave generation mechanisms

Auroral Kilometric Radiation (AKR), observed by satellites in the Earth's magnetosphere, is naturally generated in regions of partial plasma depletion (auroral density cavity) in the polar magnetosphere at approximately 3200 km altitude. As an electron descends through these regions of partial plasma depletion along magnetic field lines towards the Earth's ionosphere, the field lines increases and, through conservation of the magnetic moment, the electron gives up axial velocity in favour of perpendicular velocity. This results in a horseshoe-shaped distribution function in parallel/perpendicular-velocity space which is unstable to X-mode radiation, near the cyclotron frequency. Power levels as high as GW levels have been recorded with frequencies around 300 kHz. The background plasma frequency within the auroral density cavity is approximately 9 kHz corresponding to a plasma density 1 cm−3. A laboratory experiment scaled from auroral frequency to microwave frequency has previously been reported. Here, the addition of a Penning trap to simulate the background plasma of the density cavity is reported, with measurements ne ∼ 2 × 1014–2.17 × 1015 m−3, fpe ∼ 128–418 MHz and fce ∼ 5.21 GHz giving a ratio of ωce/ωpe comparable to the magnetospheric AKR source region.

Search for hidden periodicity in magnetometric measurements performed during auroras

A method for detecting discrete equidistant spectra in high-latitude magnetic observations is proposed. The method finds approximate solutions of the classical Schuster problem, which finds a hidden periodicity in observations with considerable noise. The usage of difference signals makes it possible to increase the latitudinal resolution of the magnetometric diagnostics. The equidistant spectra of two different types have been detected based on the proposed method. The spectra of the first type are interpreted as frequencies of coupled compressional-transverse magnetohydrodyncompressionalamic oscillations in the magnetospheric cavity. The origin of the second type of spectra can be related to a rotating source, i.e., a small-scale vortex of magnetospheric convection. Such an interpretation takes into consideration the Doppler frequency shift caused by entrainment of the ionospheric neutral gas by magnetospheric convection. The results confirm the conclusion that discrete auroral frequencies are stable under disturbed conditions.

Theoretical constraints on the generation mechanism of auroral medium frequency burst radio emissions

[1] Auroral medium frequency (MF) burst is an impulsive auroral radio emission observed at ground level between ∼1.3 and 4.5 MHz for a few minutes at substorm onsets. Despite multiple suggested theories, the exact generation mechanism for MF burst remains unknown. The foremost theory suggests that MF burst originates from Langmuir or upper hybrid waves excited over a range of altitudes in the bottomside or topside F region, which linearly mode convert to the electromagnetic LO mode, gaining access to ground level. Analysis of plasmas with two electron components implies the existence of additional normal modes, commonly referred to as the electron acoustic (EA) and electron cyclotron sound (ECS) modes, which have been suggested to potentially play a role in the generation of MF burst. Analytical calculations using Waves in Homogeneous, Anisotropic Multicomponent Plasmas, or WHAMP, and other dispersion-solving calculations are applied to applicable ionospheric normal modes (RX, LO, Langmuir or upper hybrid, EA, and ECS), compared to previous results, and applied to MF burst and the auroral plasma environment. Computation of resonant parallel energies suggests that, in the presence of an auroral electron distribution, instability of EA, ECS, LX, and Langmuir or upper hybrid modes is possible. The same analysis suggests that LO and RX modes are unlikely to be directly excited. Excitation of the EA or ECS mode in the frequency range applicable to MF burst is found to be more favorable for higher ionospheric plasma densities (fpe/fce > 2.36) and higher secondary electron temperatures (Th ≈ 100s of eV). Growth rate calculations suggest that growth of the EA or ECS mode is excitable by the auroral electron beam with higher growth rates for the ECS mode.

Ground‐based observations of diffuse auroral frequencies in the context of whistler mode chorus

We present a case study of an active auroral event containing diffuse auroral structures. These data were collected on 04 February 2009 during a ground‐based observational campaign at Poker Flat, Alaska (L ∼ 6), from January through April 2009. They were used to extract the frequencies of pulsating auroral structures that lasted many hours after substorm breakup. There are two main frequency ranges reported, coincident with the spatial resolution capabilities of the two different fields of view of the optical imagers used. The all‐sky imager clearly revealed the pulsations of the large patches, ranging from approximately 50 to 500 mHz. The narrowfield imager measured the pulsations within the finer structure to be in the range of 0.5 to 15 Hz. Both observations are consistent with the frequencies expected from theoretical, modeling and observational work on whistler mode chorus and ECH waves. In particular, the repetition frequencies observed in the visual aurora closely match the observed repetition rate of discrete chorus elements previously described in the literature. There are three significant additional insights gained through these data. The frequency of the pulsations correlates to the intensity of the aurora, with the brighter aurora containing higher frequencies. The scale size of the structures relates to the frequency of the pulsations, with the smaller structures occurring with higher frequency pulsations. The intensity profile of the pulsating structures is not symmetric, with the intensity increasing at a slower rate than it decreases.

Auroral observations in Finland: Results from all‐sky cameras, 1973–1997

Analyses of time‐space behavior of auroral occurrence were carried out using quick‐look data of Finnish all‐sky cameras operated in 1973–1997 using seven stations in Finland and one in Svalbard. This unique data set consists of 25 years of continuous auroral observations, or about 100,000 hours of all‐sky pictures. The basic parameter derived from quick‐look data is the auroral occurrence index (AO), which gives the probability (in percentage) of the occurrence of auroras during clear nights in the dark season (January to April and August to December for each year). We have analyzed the time variations of AO in diurnal, semiannual, and solar cycle timescales. The daily and semiannual occurrence of auroral frequency in Finland are consistent with the results of earlier studies carried out in other countries. There is no clear correlation between annual values of sunspot numbers and auroral occurrence within a solar cycle, but the AO values show a rather systematic increase for 1973–1993. A comparison between AO data with synoptic auroral observations made in Finland and worldwide from 1881 to 1979 reveals that both data series exhibit an increasing trend of auroral activity since the beginning of the twentieth century, similar to the global magnetic activity index series aa and sunspot number. We interpret that the increase of AO values is associated with the long‐term increase of solar magnetic activity, as has been suggested recently.

A Note on the Auroral Frequency Charts by Fritz and Vestine.

The auroral frequency charts compiled by FRITZ (1881) and VESTINE (1944), based on documentation of aurorae from AD 1700 to 1872 and that from AD 1700 to 1942, respectively, are examined in the light of secular variations in the location of the auroral zone. It is shown that the auroral zone drifted about 40° in longitude from 1700 to 1872 and a further 20° from 1872 to 1942. The auroral maximal zone in their charts is reasonably understood in terms of the westward drift of the auroral zone during the respective time intervals in which the auroral data were collected. What the auroral maximal zone in their charts represents is a photograph of the drifting auroral zone, blurred due to a long exposure time of 173 years for Ritz's chart and 243 years for Vestine's chart.

Aurora borealis lags during the Middle Ages

One thousand five hundred and thirty-eight days for which Northern Lights at middle latitudes below 55°N were recorded, from the 13th to mid-18th centuries, are analysed statistically with respect to the phase relationship between sunspot and aurora borealis cycles. Proof of a phase-dependent frequency of aurorae boreales is presented for parts of this period, which includes epochs of reduced and normal solar activity. For example, the auroral maxima in the Maunder and Sprer minima coincide with the lag in auroral frequency discovered by Fritz [(1864) Vierteljahresschrift der Naturforschenden Gesellschaft Zürich 4, 122]. Such a dependence cannot be demonstrated for the preceding Wolf minimum.

Hemispherical asymmetries in sunspot areas and auroral frequencies

One thousand and fifty-two aurorae boreales and 554 aurorae australes recorded during the nineteenth century at medium latitudes ≦ 55° N or ≦ 55° S are compared statistically with the known hemispherical asymmetry of the sums of the areas of sunspots. According to the present study, the solar hemispherical asymmetry may be accompanied by an analogous pattern of the hemispherical frequency of auroral days. For the number of auroral days in each hemisphere beyond the two auroral ovals, a remarkable degree of phase equality with the sunspot areas during the second Gleissberg cycle can be demonstrated.

Evidence for periodicities in the auroral occurrence frequency since 300 AD and their implications

Spectral analyses of the Chinese auroral records dating from 300 AD using maximum entropy, Fourier and autocorrelation methods have revealed significant periods. The power spectral density of the auroral records has a strong multiplet of spectral lines centered on the median period. The prominent period is 200 years followed by enveloping bands of 80–130, 40–60 and 20–30 years. Nonstationarity is found for periods less than 120 years. The similarity of these periods with intervals between sunspot minima and other solar-terrestrial parameters is noted

Latitude dependence of auroral frequency in relation to solar — Terrestrial and interplanetary parameters

The auroral frequency of occurrences (A) for the 20th solar cycle and for the geomagnetic latitudes 54‡–63‡ N has been investigated in relation to sunspot numbers (Rz), number of flares (F), the solar wind streams derived from the coronal holes (H) and the geomagnetic index (Ap). The relationship between A and the other indices were found to be strongly latitude dependent. At around 57‡–58‡ N, a drastic change in this relationship occurs, and an attempt is made qualitatively to evaluate this latitudinal variation.

Nocturnal ground irradiance at high latitudes

Measurements of the nocturnal ground irradiance in the 0.4–1-μm spectral region during clear weather from five different sites covering geographic latitudes of 60–80°N are reported. The aurora dominates the ground irradiance. For latitudes from the auroral zone and south a diurnal variation with maximum irradiance around geomagnetic midnight is found. The relative variation of ground irradiance vs latitude follows closely the average auroral occurrence frequency except for extreme high latitudes. In the auroral zone region an average irradiance level of 3.8 μW cm−2 sr−1μm−1 × 10−3 is found. This about eight times the corresponding ground irradiance outside auroral regions.

Auroral frequency in the 17th and 18th centuries and the ‘Maunder minimum’

It is shown that such data as exist on auroral observations in the latter half of the 17th century do not provide the conclusive evidence that some authors have claimed for the so-called Maunder minimum in solar activity between 1645 and 1715.

Differences in auroral intensity at conjugate points

Conjugate auroral all-sky camera data obtained in 18 flights through the auroral zone near the College, Alaska, magnetic meridian show hemispherical differences in auroral frequency and intensity. Conjugate auroras on the equatorward boundary of the display (the equatorward arc system) are found to be consistently brighter in the northern hemisphere by a factor of approximately 1.3. At higher latitudes (in the poleward arc system) the auroras exhibit varying degrees of conjugacy, but the auroras in the north tend to occur more frequently and to be brighter. The hemispherical difference is attributed to the 8000-γ difference in magnetic field strength between the conjugate areas. Simplified theoretical calculations of the difference in precipitation at conjugate points from idealized cases of pitch angle diffusion and from a small hemispherical potential difference show that pitch angle diffusion may explain observations in the equatorward arc system but not in the poleward arc system. The evidence for hemispheric differences in auroral particle precipitation adds substantially to earlier indications of variation with longitude of auroral precipitation patterns.

Remarks on the Seasonal Changes of Temperature and of Ozone in the Arctic and the Antarctic Stratospheres

The seasonal regimes of temperature in the arctic and the antarctic stratospheres are contrasted, with special reference to the sporadic winter warmings of the arctic and the explosive spring warming of the antarctic stratospheres. There follows a discussion of the inadequacy of the usual advective-dynamic explanation, at least in terms of any model presented to date, to account quantitatively for any of these phenomena in their more extreme manifestations. The seasonal regimes of total atmospheric ozone in the arctic and in the antarctic are likewise contrasted, again with special reference to the differing thermal regimes and sudden stratospheric warming. The strong annual period of auroral frequencies and the belts of maximum auroral frequency are discussed in relation to the arctic and the antarctic thermal regimes as is the relationship of sudden stratospheric warmings to auroral and solar activity. Finally, variability of the solar wind, i.e., of solar corpuscular penetration of the higher atmosphere, is tentatively suggested as a possible alternative explanation to the advective-dynamic hypothesis to account for the variation in common of polar stratospheric temperature, ozone and auroral activity.

The interpretation of the frequency dependence of radio aurora

Previous attempts to explain observations of the frequency dependence of radioaurora have involved the introduction of critical reflection at V.H.F. This implies electron densities which are considered by some to be unreasonably high. The interpretation of the frequency dependence of radio-aurora is re-examined in this paper, and it is shown that a random assembly of diffusing scatterers, which requires critical reflection and other effects for an explanation of the frequency dependence, is an unsatisfactory model of radio-auroral ionization for several reasons. A wave-generating process, such as a two-stream instability associated with the (auroral) electrojet, appears to provide the most reasonable explanation of the observations of radio auroral frequency dependence.