On oscillations found in an active region with EIS on Hinode

Abstract

Aims. Using temporal series (40 �� slot) data in the Fe xii 195 A line from the extreme-ultraviolet imaging spectrometer (EIS) on board the Hinode satellite, we seek to carry out a statistical study of oscillations in an active region. Methods. Using wavelet techniques, we measure the frequency and duration of oscillations found in the time series data. Stokes I and V data from Hinode’s SOT are used to measure photospheric magnetic fields, while an EIS raster is used to provide estimates of LOS velocity and electron density. The measured frequencies are displayed as maps in different bands to aid their analysis. Results. Oscillations over a broad range of frequencies (2–154 mHz) are found throughout the active region at the temperature of Fe xii (log T = 6.1 K). Oscillations with frequencies between 2–8 mHz are concentrated in bright plage areas, while oscillations of higher frequency preferentially group at the edges of these areas. Evidence is found for harmonics on the boundary of the active region loop, indicating the presence of standing waves. From a measurement of the lengths and electron density of loops appearing in our active region field-of-view (FOV), together with a knowledge of the periods (frequencies) of the oscillations within the area of these loops, we find a magnetic field value of between 12.2 ± 2.08−18.9 ± 3.40 G, assuming standing fast kink waves. Conclusions. We conclude that waves, which the observed oscillations are signatures of, are common in the active region studied, over a broad range of frequencies. The change in the distribution of the oscillations present at frequencies less than 8 mHz and those at all frequencies above this indicates a variation of the wave mode with frequency, e.g., possibly from slow magnetoacoustic waves in the bright plage regions to fast magnetoacoustic waves at the boundaries of these regions. The preference for higher frequency oscillations to cluster on the edges of regions of high magnetic field is considered indicative of resonant absorption. Based on our measurements of magnetic field, we conclude that the oscillations found by us in the active region loop are fast kink waves. Previous studies such as O’Shea et al. (2001 )a ndIreland et al. (1999) carried out statistical studies of the oscillatory characteristics of active regions using transition region and coronal lines observed with CDS on board the SOHO spacecraft. Here, we seek to carry out a similar statistical study but with the superior temporal resolution and imaging capabilities of the EUV imaging spectrometer (EIS) on board the Hinode spacecraft (Culhane et al. 2007). In this work we will present a global statistical analysis of all intensity oscillations, measured in a Fe xii 195 A line, present in an active region on the disk. From this statistical study we will seek to identify different wave modes present in the active region and its structures. We note that O’Shea et al. (2001) found evidence for non-compressive (transverse) waves to be more prevalent in coronal lines (at temperatures between Log T = 6.0−6.3 K) than in lower temperatures lines. We also, therefore, seek to find further evidence for these non-compressive waves. From the measured oscillation frequencies (periods), together with the observed physical quantities of loop length, radius and electron density, we will seek to provide estimates of the magnetic field in the corona. We note that Nakariakov & Ofman (2001), using a similar technique to that which will be used by us, obtained values of 13 ± 9 G for global standing kink waves in an active region.