On the Spatial Dependence of Low-Degree Solar p -Mode Frequency Shifts from Full-Disk and Resolved-Sun Observations

Abstract

We have analyzed low angular degree (low-) solar p-mode frequency shifts extracted from two types of ᐉ observations, both made by instruments on board the ESA/NASA Solar and Heliospheric Observatory satellite during a period that covers the rising phase of activity, cycle 23 (1995-2002). The first are full-disk (Sun-as-astar) averages made in Doppler velocity by the Global Oscillations at Low Frequency spectrophotometer; the second are Doppler velocity observations made with high spatial resolution by the Michelson Doppler Imager. We compare the eigenfrequency shifts from both sets of data and find that they are consistent at the level of precision of the observations. Furthermore, the sizes of the shifts uncovered for different mode components are found to scale in proportion to the corresponding spherical harmonic components of the observed line-of-sight surface magnetic field, with the sectoral mode components showing (as expected) the largest shifts.

The onset of solar cycle 24

“…Jiménez-Reyes et al (2004) previously showed that the values of the frequency shifts for l = 0, 1, and 2 measured in MDI data scale to the corresponding spherical harmonic components of the observed lineof-sight surface magnetic field. The analysis of the variability in the other p-mode parameters during this extended minimum is also currently underway.…”

“…A marginal but significant l dependence of the frequency shifts of the low-degree p modes has also been clearly identified (Jiménez-Reyes et al 2004;Chaplin et al 2004). The size of the shift also reflects differences in mode inertia.…”

“…This is because the solar rotation axis lies close to the plane of the sky from the observing point. The observed l = 1 frequency is therefore a weighted measurement of the visible components (l = 1, |m| = 1), while in the case of the l = 2 mode, the fitted frequency corresponds to the weighted measurement of the zonal (l = 2, m = 0) and sectoral (l = 2, |m| = 2) components (Jiménez-Reyes et al 2004). The sectoral modes are more concentrated along the equator, while the zonal modes are most sensitive to the high latitudes.…”

“…While solar p-mode frequency shifts have been shown to be closely correlated with the solar surface activity during previous solar cycles (Howe et al 2002;Jiménez-Reyes et al 2004;Chaplin et al 2007), it appears that during the unpredicted long minimum of cycle 23, the frequency shifts of the low-degree p modes have exhibited unexpected behavior. Since the second half of the year 2007, the frequency shifts have been observed to be uncorrelated with surface activity.…”

“…Later on, Woodard & Libbrecht (1991) measured similar variations in intermediate and high angular-degree modes. As longer, higher quality, and continuous helioseismic observations became available, the solar p-mode frequencies proved to be very sensitive to the solar surface activity not only during solar cycle 23 (see for example, Howe et al 2002;Jiménez-Reyes et al 2004, and references therein) but also over the last three solar cycles (21, 22, and 23), spanning a period of thirty years , with high levels of correlation with solar surface activity proxies.…”