Abstract: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 (1998a,b). 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 18 th century, solar activity had already returned to levels equivalent to those observed in recent solar cycles in the 20 th 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 21 st century.

We discuss recent papers very critical of our Group Sunspot Number Series (Svalgaard & Schatten [2016]). Unfortunately, we cannot support any of the concerns they raise. We first show that almost always there is simple proportionality between the group counts by different observers and that taking the small, occasional, non-linearities into account makes very little difference. Among other examples: we verify that the RGO group count was drifting the first twenty years of observations. We then show that our group count matches the diurnal variation of the geomagnetic field with high fidelity, and that the heliospheric magnetic field derived from geomagnetic data is consistent with our group number series. We evaluate the 'correction matrix' approach ] and show that it fails to reproduce the observational data. We clarify the notion of daisychaining and point out that our group number series has no daisy-chaining for the period 1794-1996 and therefore no accumulation of errors over that span. We compare with the cosmic ray record for the last 400+ years and find good agreement. We note that the Active Day Fraction method (of Usoskin et al.) has the fundamental problem that at sunspot maximum, every day is an 'active day' so ADF is nearly always unity and thus does not carry information about the statistics of high solar activity. This 'information shadow' occurs for even moderate group numbers and thus need to be extrapolated to higher activity. The ADF method also fails for 'equivalent observers' who should register the same group counts, but do not. We conclude that the criticism of Svalgaard & Schatten [2016] is invalid and detrimental to progress in the important field of long-term variation of solar activity.

We describe a revised collection of the number of sunspot groups from 1610 to the present. This new collection is based on the work of Hoyt and Schatten (Solar Phys. 179, 189, 1998). The main changes are the elimination of a considerable number of observations during the Maunder Minimum (hereafter, MM) and the inclusion of several long series of observations. Numerous minor changes are also described. Moreover, we have calculated the active-day percentage during the MM from this new collection as a reliable index of the solar activity. Thus, the level of solar activity obtained in this work is greater than the level obtained using the original Hoyt and Schatten data, although it remains compatible with a grand minimum of solar activity. The new collection is available in digital format.Comment: 27 pages, 6 figures, accepted for publication in Solar Physic

On physical grounds it is suggested that the sun's polar field strength near a solar minimum is closely related to the following cycle's solar activity. Four methods of estimating the sun's polar magnetic field strength near solar minimum are employed to provide an estimate of cycle 21's yearly mean sunspot number at solar maximum of 140 ± 20. We think of this estimate as a first order attempt to predict the cycle's activity using one parameter of physical importance based upon dynamo theory.

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of Information. Send comments regarding this burden estimate or any other aspect of this collection of information, Including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 14. ABSTRACT Abstract. It is generally appreciated that the September 1859 solar-terrestrial disturbance, the first recognized space weather event, was exceptionally large. How' large and how exceptional? To answer these questions, we compiled rank order lists of the various measures of solar-induced disturbance for events from 1859 to the present. The parameter~s Considered included: magnetic crochet amplitude, solar energetic proton fluence (McCracken et aL, 2001 a), Sun-Earth disturbance transit time, geomagnetic storm intensity, and low-latitude auroral extent. While the 1859 event has close rivals or superiors in each of the above categories of space weather activity, it is the only documented event of the last -150 years that appears at or near the top of all of the lists. Taken together, the top-ranking events in each of the disturbance categories comprise a set of benchmarks for extreme space weather activity. Abstract. It is generally appreciated that the September 1859 solar-terrestrial disturbance, the first recognized space weather event, was exceptionally large. How large and how exceptional? To answer these questions, we compiled rank order lists of the various measures of solar-induced disturbance for events from 1859 to the present. The parameters considered included: magnetic crochet amplitude, solar energetic proton fluence (McCracken et al., 2001a), Sun-Earth disturbance transit time, geomagnetic storm intensity, and low-latitude auroral extent. While the 1859 event has close rivals or superiors in each of the above categories of space weather activity, it is the only documented event of the last -150 years that appears at or near the top of all of the lists. Taken together, the top-ranking events in each of the disturbance categories comprise a set of benchmarks for extreme space weather activity. REPORT DATE (DD-MM-YYYY) SPONSORING I MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S) AFRL/VSBXS SPONSOR/MONITOR'S REPORT NUMBER(S)AFRL SUBJECT

[1] On the basis of a consideration of Bartels' historical u index of geomagnetic activity, we devise an equivalent index that we refer to as the interdiurnal variability (IDV). The IDV index has the interesting and useful property of being highly correlated with the strength of the interplanetary magnetic field (B; R 2 = 0.75) and essentially unaffected by the solar wind speed (V; R 2 = 0.01) as measured by spacecraft. This enables us to obtain the variation of B from 1872 to the present, providing an independent check on previously reported results for the evolution of this parameter. We find that solar cycle average B increased by $25% from the 1900s to the 1950s and has been lower since. If predictions for a small solar cycle 24 bear out, solar cycle average B will return to levels of $100 years ago during the coming cycle(s).

[1] 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°t ilt of the solar equator to the ecliptic plane. Using direct polar field measurements, now available for four solar cycles, we predict that the approaching solar cycle 24 ($2011 maximum) will have a peak smoothed monthly sunspot number of 75 ± 8, making it potentially the smallest cycle in the last 100 years.

IntroductionThe magnetic field strength within the polar caps, of the sun is an important parameter for both the solar activity cycle and for our understanding of the interplanetary magnetic field.Measurements In addition, extensive background fields are apparent including welldefined polar fields. Referring to Figure 1 we shall be particularly interested in the field measured in the polar cap, i.e. the polemost scan line in each hemisphere. The average latitude of the equatoro ward limit of that aperture is 55 . At the present time that is also the equatorward boundary of the polar coronal holes (Solar Geophysical Data, 1976. In accordance with that, the measured polar fields have invariably been unipolar (over the aperture) throughout the interval covered by the daily magnetograms -16 May 1976 through August 1977 -positive or outwards in the north and negative or inwards in the southern polar cap. The position of an aperture along the scan line is measured by the parameter x, being 0 at central meridian, and ±11 respectively at the west -and east equatorial limbs ( Figure 1). We shall be concerned with apertures at x = 0 (black in Figure 1) and at x = ±2 (dotted areas in Figure 1).During the course of a year the solar rotation axis tips toward and later away from the observer by l\ . We thus get a kind of stereoscopic view of the polar fields provided that they do not change appreciably over a timescale of up to a year.In order to interpret measurements of the polar cap fields, it is necessary to investigate if the fact thtt the data is taken near the limb is distorting the data in any way. It has been suggested by Howard and Stenflo (1972) that measurements of the magnetic field in the \525.02 nm line are systematically in error by a factor that 2 depends on the angle between the radius and the line of sight.This error changes by more than a factor of two as one goes from the center of the disk to the limb. By following specific areas as they cross the disk from east limb to west limb in the course of solar rotation we may directly verify the existence of this systematic error under the weak assumption that the intrinsic field does not change considerably during disk passage. In the following section we report the results of an extensive analysis of measured field strengths as a function of the distance from disk center. Our conclusion is that no systematic error of the kind suggested by Howard and Stenflo (1972) is apparent in the Stanford data. Center to limb variation of field strengthWhen an aperture is placed at central meridian an area is defined on the solar surface. The area can be assigned heliographic coordinates and can be identified on magnetograms both before and after central Figure 4 shows the average B« as a function of cosL. The analysis was performed separately for weak fields (|B| < 150 n,T) and for strong fields (lB|i 150 tiT). The magnetic field strength at central meridian passage was used as the selection parameter. Furthermore, the data was analyzed separately for each polarity of ...