Edward W. Cliver

Sunspot cycle 24: Smallest cycle in 100 years?

Abstract : Predicting the peak amplitude of the sunspot cycle is a key goal of solar-terrestrial physics. The precursor method currently favored for such predictions is based on the dynamo model in which large-scale polar fields on the decline of the 11-year solar cycle are converted to toroidal (sunspot) fields during the subsequent cycle. The strength of the polar fields during the decay of one cycle is assumed to be an indicator of peak sunspot activity for the following cycle. Polar fields reach their peak amplitude several years after sunspot maximum; the time of peak strength is signaled by the onset of a strong annual modulation of polar fields due to the 7 1/4-degree tilt of the solar equator to the ecliptic plane. Using direct polar field measurements, now available for four solar cycles, the authors predict that the approaching solar cycle 24 (approx. 2011 maximum) will have a peak smoothed monthly sunspot number of 75 +/- 8, making it potentially the smallest cycle in the last 100 years.

Mountains versus valleys: Semiannual variation of geomagnetic activity

The semiannual variation in geomagnetic activity is generally attributed to the Russell-McPherron effect. In that picture, enhancements of southward field Bs, near the equinoxes account for the observed higher geomagnetic activity in March and September. In a contrary point of view, we argue that the bulk of the semiannual variation results from an equinoctial effect (based on the Ψ angle between the solar wind flow direction and Earths dipole axis) that makes Bs, coupling less effective (by ∼25% on average) at the solstices. Thus the semiannual variation is not simply due to “mountain building” (creation of Bs) at the equinoxes but results primarily from “valley digging” (loss of coupling efficiency) at the solstices. We estimate that this latter effect, which clearly reveals itself in the diurnal variation of the am index, is responsible for ∼65% of the semiannual modulation. The characteristic imprint of the equinoctial hypothesis is also apparent in hourly/monthly averages of the time-differentiated Dst index and the AE index.

Revisiting the Sunspot Number

Our knowledge of the long-term evolution of solar activity and of its primary modulation, the 11-year cycle, largely depends on a single direct observational record: the visual sunspot counts that retrace the last 4 centuries, since the invention of the astronomical telescope. Currently, this activity index is available in two main forms: the International Sunspot Number initiated by R. Wolf in 1849 and the Group Number constructed more recently by Hoyt and Schatten (Sol. Phys. 179:189–219, 1998a, 181:491–512, 1998b). Unfortunately, those two series do not match by various aspects, inducing confusions and contradictions when used in crucial contemporary studies of the solar dynamo or of the solar forcing on the Earth climate. Recently, new efforts have been undertaken to diagnose and correct flaws and biases affecting both sunspot series, in the framework of a series of dedicated Sunspot Number Workshops. Here, we present a global overview of our current understanding of the sunspot number calibration.After retracing the construction of those two composite series, we present the new concepts and methods used to self-consistently re-calibrate the original sunspot series. While the early part of the sunspot record before 1800 is still characterized by large uncertainties due to poorly observed periods, the more recent sunspot numbers are mainly affected by three main inhomogeneities: in 1880–1915 for the Group Number and in 1947 and 1980–2014 for the Sunspot Number.After establishing those new corrections, we then consider the implications on our knowledge of solar activity over the last 400 years. The newly corrected series clearly indicates a progressive decline of solar activity before the onset of the Maunder Minimum, while the slowly rising trend of the activity after the Maunder Minimum is strongly reduced, suggesting that by the mid 18th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20th century. We finally conclude with future prospects opened by this epochal revision of the Sunspot Number, the first one since Wolf himself, and its reconciliation with the Group Number, a long-awaited modernization that will feed solar cycle research into the 21st century.

CORONAL SHOCKS AND SOLAR ENERGETIC PROTON EVENTS

From 1996 July through 2001 June, less than half (43/98) of all favorably located (from solar western hemisphere sources) metric type II radio bursts were associated with solar energetic proton (SEP) events observed at Earth. When western hemisphere metric type IIs were accompanied by decametric-hectometric (DH; 1-14 MHz) type II emission (observed by Wind/WAVES) during this period, their association with ~20 MeV SEP events (with peak fluxes ≥10-3 protons cm-2 s-1 sr-1 MeV-1) was 90% (26/29), versus only 25% (17/69) for metric IIs without a DH counterpart. Overall, 82% (63%) of all SEP events with visible disk origins were associated with metric (DH) type II bursts, with the percentage associations increasing with SEP event size to 88% (96%) for ~20 MeV SEP events with peak intensities of ≥10-1 protons cm-2 s-1 sr-1 MeV-1. Our results are consistent with the following possibilities (which are not mutually exclusive): (1) large ~20 MeV SEP events result from strong shocks that are capable of persisting well beyond ~3 R☉ (the nominal 14 MHz plasma level); (2) shock acceleration is most efficient above ~3 R☉; and (3) shocks that survive beyond ~3 R☉ are more likely to have broad longitudinal extents, enabling less well connected shocks to intercept open field lines connecting to Earth.

INJECTION ONSETS OF approximately. 2-GEV PROTONS, approximately 1-MEV ELECTRONS, AND approximately 100-KEV ELECTRONS IN SOLAR COSMIC RAY FLARES

We review the data for all 32 ground-level cosmic-ray events (GLEs) observed from 1942 through 1978 and infer injection onset times for the approx.2 GeV protons, approx.1 MeV electrons, and approx.100 keV electrons. Contrary to previous investigations, we find no compelling evidence for a systematic delay in GLE onset times. The most likely time of GeV proton injection onset in these large flares appears to be near the maximum of the first significant microwave peak. We note that GLEs with long delays to onset tend to be small in size. In addition, the data indicate a systematic phase relationship among the injection onsets of the three particle species considered, with the low-energy electron onset times proceding those of the relativistic protons by or approx. =5 min. This phase relationship holds even when the inferred injection times of all three species follow the flare flash phase by >20 min. To account for these observations, we suggest a picture in which the earliest observed particles are injected when an outward moving acceleration region at a shock front intersects the open field lines connecting to Earth.

The 22‐year cycle of geomagnetic and solar wind activity

The 22-year cycle in geomagnetic activity is characterized by high activity during the second half of even-numbered solar cycles and the first half of odd-numbered cycles. We present new evidence for this 22-year cycle using the aa magnetic index for the years 1844–1994. Over this 150-year interval, the 22-year cycle can be observed through differences between the decay phases of even- and odd-numbered cycles in (1) average values of a 27-day recurrence index; (2) the results of a χ2 “event” analysis of 27-day recurrences of both disturbed and quiet days; and (3) an apparent annual modulation of the 27-day peak in the power spectrum of the aa index. Currently, the 22-year variation is attributed to the Russell-McPherron solar wind - magnetosphere coupling mechanism working in conjunction with the Rosenberg-Coleman polarity effect. Contrary to this viewpoint, we argue that an intrinsic 22-year solar variation (other than polarity reversal), revealed in the systematic low-high alternation of even-odd sunspot maxima within the last six complete Hale cycles, is the dominant cause of the 22-year cycle in geomagnetic activity. This sunspot and related coronal mass ejection variation should lead directly to higher geomagnetic activity during the first-half of odd-numbered solar cycles. Various lines of evidence (including 1–3 above) indicate that 27-day recurrent wind streams are more prominent during the decline of even-numbered solar cycles, contributing to the higher geomagnetic activity observed at those times. These stronger recurrence patterns may be related to the more rapid expansion of polar coronal holes (faster movement of the coronal streamer belt to low latitudes) observed following the maxima of recent even-numbered cycles. The amplitudes of the 22-year sunspot and geomagnetic activity cycles over the last 150 years are shown to be highly correlated. The 22-year pattern of geomagnetic activity appears to be a reflection of the solar dynamo coupling of poloidal magnetic fields on the decline of one solar cycle to the toroidal fields at the maximum of the following cycle. It seems likely that the 22-year variation in sunspot/solar wind activity plays a role in the observed 22-year modulation of galactic cosmic ray intensity.

Observing coronal mass ejections without coronagraphs

A coronal mass ejection (CME), strictly speaking, is a phenomenon observed via a white-light coronal imager. In addition to coronagraphs, a wide variety of other instruments provide independent observations of CMEs, in regimes ranging from the chromosphere to interplanetary space. In this paper we list the most important of these noncoronagraphic signatures, many of which had been known even before CMEs were first identified in coronagraph observations about 30 years ago. We summarize the new aspects of CMEs discovered in the past several years, primarily with instruments on the Yohkoh and SOHO satellites. We emphasize the need for detailed statistically based comparisons between SOHO CMEs and their noncoronagraphic manifestations. We discuss how the various aspects of CMEs fit into the current standard model (sigmoids, flux rope, double dimming, arcade). While a class of CMEs follows this pattern, it does not appear to work for all events. In particular, some CMEs involve extended dimming regions and erupting transequatorial X-ray loops, indicating a more complex geometry than a simple bipolar magnetic configuration.

Injection onsets of approx. 2GeV protons, approx. 1 MeV electrons, and approx. 100 keV electrons in solar cosmic ray flares

We review the data for all 32 ground-level cosmic-ray events (GLEs) observed from 1942 through 1978 and infer injection onset times for the approx.2 GeV protons, approx.1 MeV electrons, and approx.100 keV electrons. Contrary to previous investigations, we find no compelling evidence for a systematic delay in GLE onset times. The most likely time of GeV proton injection onset in these large flares appears to be near the maximum of the first significant microwave peak. We note that GLEs with long delays to onset tend to be small in size. In addition, the data indicate a systematic phase relationship among the injection onsets of the three particle species considered, with the low-energy electron onset times proceding those of the relativistic protons by or approx. =5 min. This phase relationship holds even when the inferred injection times of all three species follow the flare flash phase by >20 min. To account for these observations, we suggest a picture in which the earliest observed particles are injected when an outward moving acceleration region at a shock front intersects the open field lines connecting to Earth.

22 Year Patterns in the Relationship of Sunspot Number and Tilt Angle to Cosmic-Ray Intensity

A comparison of 27 day averages of the sunspot number with the Galactic cosmic-ray intensity observed at Climax reveals a 22 yr pattern. The 11 yr cosmic-ray cycle appears to lag the sunspot cycle by ~1 yr for odd-numbered cycles such as 19 and 21. During even-numbered cycles the sunspot number and cosmic-ray intensity curves are essentially in phase. A similar pattern is apparent in a comparison of the tilt angle of the heliospheric current sheet (HCS) with cosmic-ray intensity for the last three solar cycles (21-23). The tilt angle evolution on the rise of the last three cycles was remarkably similar, while the decline of the tilt angle from high values at the maximum of cycle 21 (~1980) was more gradual than that observed following the maximum of cycle 22 (~1990) or that inferred from coronal hole areas for cycle 20 (~1970). The reduced responsiveness of cosmic rays to sunspot or tilt angle increases on the rise of odd-numbered solar cycles is consistent with a drift effect. A difference in the evolution of large-scale fields on the decay of even- and odd-numbered cycles may contribute to more gradual recovery of cosmic-ray intensity following the maxima of odd-numbered cycles. The onset of modulation in odd-numbered cycles, and of diffusion/convection-dominated modulation in even-numbered cycles, appears to begin when the tilt angle of the HCS exceeds ~50°.

Geomagnetic activity and the solar wind during the Maunder Minimum

We used a strong (r = 0.96) correlation between 11-year averages of sunspot number (SSN) and the geomagnetic aa index to infer that the mean level of geomagnetic activity during the Maunder Minimum (1645–1715) was approximately a third of that observed for recent solar cycles (∼7 nT vs. ∼24 nT). We determined the variation of 11-year averages of solar wind speed (v) and the southward component of the interplanetary magnetic field (Bs) with cycle-averaged SSN for the two most recent cycles and also compared cycle-averaged variations of v²Bs and aa for the same interval. We then extrapolated these observed solar wind variations to Maunder Minimum conditions (mean SSN of ∼ 2 and mean aa value of ∼ 7 nT) to deduce that, on average, the solar wind during that period was somewhat slower (v = 340 ± 50 km s−1), and the interplanetary magnetic field much smoother (Bs = 0.3±0.1 nT), than at present (∼ 440 km s−1 and ∼ 1.2 nT). Various lines of evidence (including 10Be data) suggest that, despite the virtual absence of sunspots that characterized the Maunder Minimum, the 11-year geomagnetic (solar wind) cycle persisted throughout this period.