Th. Ulich

A new method to determine the solar cycle length

The length of the solar cycle and its long-term variation have recently received additional significance due to their suggested connection to global climate. The cycle length is conventionally defined as the time difference between two successive sunspot minima. However, the sunspot minimum times sensitively depend on the way the sunspot numbers are averaged, i.e. whether one uses daily or monthly averaged values and whether and how the data are further smoothed. Using differently processed sunspot data, the sunspot minimum times vary typically by a few months, leading to a corresponding inaccuracy in solar cycle length. Here we propose a new method to define the solar cycle length as a difference between the median activity times of two successive sunspot cycles. The great advantage of this method is that the median times are almost independent of how the sunspot minima are determined. Therefore the method allows the solar cycle lengths to be calculated with a very small inaccuracy of a few days only. We show that the individual cycle lengths calculated from the conventional and the median method may differ by nearly a year. However, the long-term trend of cycle lengths remains roughly the same during modern times.

Methodological influences on F-region peak height trend analyses

Published estimates of the trend in hmF2 using data from ionosondes over the last 30-40 years range from +0.8 to -0.6 km yr(-1) and are subject to the influence of several factors. These are considered here based upon an analysis of two southern hemisphere geomagnetically mid-latitude stations, Argentine Islands and Port Stanley. The influence of the equation used to calculate hmF2 at these stations can result in variations of +/-0.2 km yr(-1); choice of solar proxy has a small influence on the end result, where using E10.7 instead of F10.7 produces changes of -0.04 km yr(-1); neglecting any trends in geomagnetic activity can produce variations of +0.03 to +0.2 km yr(-1) at the two mid-latitude stations considered in this paper; for datasets of 30-40 years length ringing due to long memory processes can produce +/-0.2 km yr(-1) variability; the phase of the 11-year solar cycle, and its harmonics, captured by the datasets can cause variability of +/-0.5 km yr(-1); and the neglect of local time variations in thermospheric wind conditions could result in +0.2 km yr(-1) for analysis which only considers local midday data. The Argentine Islands and Port Stanley datasets show ringing terms that are still converging towards trend results of -0.25 to -0.30 km yr(-1), which are in close agreement with the satellite drag trend estimates.

Broadband meter‐wavelength observations of ionospheric scintillation

Intensity scintillations of cosmic radio sources are used to study astrophysical plasmas like the ionosphere, the solar wind, and the interstellar medium. Normally, these observations are relatively narrow band. With Low-Frequency Array (LOFAR) technology at the Kilpisjarvi Atmospheric Imaging Receiver Array (KAIRA) station in northern Finland we have observed scintillations over a three-octave bandwidth. “Parabolic arcs,” which were discovered in interstellar scintillations of pulsars, can provide precise estimates of the distance and velocity of the scattering plasma. Here we report the first observations of such arcs in the ionosphere and the first broadband observations of arcs anywhere, raising hopes that study of the phenomenon may similarly improve the analysis of ionospheric scintillations. These observations were made of the strong natural radio source Cygnus-A and covered the entire 30–250 MHz band of KAIRA. Well-defined parabolic arcs were seen early in the observations, before transit, and disappeared after transit although scintillations continued to be obvious during the entire observation. We show that this can be attributed to the structure of Cygnus-A. Initial results from modeling these scintillation arcs are consistent with simultaneous ionospheric soundings taken with other instruments and indicate that scattering is most likely to be associated more with the topside ionosphere than the F region peak altitude. Further modeling and possible extension to interferometric observations, using international LOFAR stations, are discussed.

First optical observations of energetic electron precipitation at 4278 Å caused by a powerful VLF transmitter

A summary is presented of experimental optical observations at 4278 A from close to a powerful (~150 kW) VLF transmitter (call sign JXN) with a transmission frequency of 16.4 kHz. Approximately 2.5 s after transmitter turn-on, a sudden increase in optical emissions at 4278 A was detected using a dedicated camera/charge-coupled device (CCD) monitoring system recording at a frequency of 10 Hz. The optical signal is interpreted as a burst of electron precipitation lasting ~0.5 s, due to gyro-resonant wave-particle interactions between the transmitted wave and the magnetospheric electron population. The precipitation was centered on the zenith and had no detectable spatial structure. The timing of this sequence of events is in line with theoretical predictions and previous indirect observations of precipitation. This first direct measurement of VLF-induced precipitation at 4278 A reveals the spatial and temporal extent of the resulting optical signal close to the transmitter.

Northern Hemisphere Stratospheric Ozone Depletion Caused by Solar Proton Events: The Role of the Polar Vortex

Ozonesonde data from four sites are analyzed in relation to 191 solar protons events (SPEs) from 1989-2016. Analysis shows ozone depletion (~10-35 km altitude) commencing following the SPEs. Seasonally-corrected ozone data demonstrate that depletions occur only in winter/early-spring above sites where the northern hemisphere polar vortex (PV) can be present. A rapid reduction in stratospheric ozone is observed with the maximum decrease occurring ~10-20 days after SPEs. Ozone levels remain depleted in excess of 30 days. No depletion is observed above sites completely outside the PV. No depletion is observed in relation to 191 random epochs at any site at any time of year. Results point to the role of indirect ozone destruction, most likely via the rapid descent of long-lived NOx species in the PV during the polar winter.